Not only am I raising my own voice for kicking Dread Necro and Beguiler up a tier, but I have a question too. What is the Class X fallacy?
The Class X fallacy is when someone argues that one class is more powerful relative to other classes because of something that applies to all classes. A good example is "Fighters are powerful and versitile, because they could take cross class UMD ranks to be able to use spellcaster abilities!" However, the given reason would still be true if you replaced the word "Fighters" in that scentence with any other class... the statement "[Class X] are powerful and versitile, because they could take..." applies just as well. Hence the name.
And yes, I've seen that very argument. If you've read that rather comical Monk guide, it used the same thing, only with Monks. A very common one though is "[Class X] is really strong if a Wizard is there to buff it" or "[Class X] is really strong, since all it has to do is be this powerful race and have these powerful (and non class specific) feats." Arguing that the Commoner is a powerful class because a Black Ethergaunt Commoner 1 is clearly Tier 1 would be a great example.
As for the Beguiler and Dread Necromancer, I've got good reason to keep them below the Sorcerer and such in terms of raw power, but I will say that the first time I made this system Tier 2 and 3 were merged. After all, what the Beguiler lacks in super powered spells it often makes up for in useful class features and skills, and it does have some seriously potent spells in its arsinal. The same can be said of Dread Necromancers. But playing the three classes (and often with others of the three in the same group) I can safely say that there's a distinct power differential, and it's especially noticeable at higher levels. I'll give an example.
One game I'm in right now originally had a Dread Necromancer and a Beguiler in the party, starting at level 2 and playing up through level 6. It also had a Swordsage (technically, a variant that had sudden strike and traded out some manuevers, but it was close enough) and a Paladin of Tyranny/Hexblade, and yes we did intentionally play Tier 3. It was even pretty noticeable that that last guy was the odd one out... we kept having to leave him behind during stealth and recon missions, because he would have been more of a liability than an asset. And when we fought, we fought as a great team... the Beguiler dropped Glitterdusts while the Dread Necromancer used Fear Aura and Intimidate, and the Swordsage did precision damage, while the PoT ensured few enemies made their save. All fine and well. As we leveled up, the Beguiler player got bored (mostly because the Beguiler just didn't fit his strengths, he was more of a hitty player) and let his character get killed. Still, it was pretty obvious that all he was doing was spamming Glitterdust all the time... an effective technique, but one easily duplicated by a Sorcerer. His one big advantage was Mindsight, which he got right before he died. Then I raised him and now I play him, though the DM won't let him level up so he's stuck at level 6.
Anyway, the Beguiler player switched to a Cleric and then soon left the game, but at the same time a new player came in with a Sorcerer. She too would spam Glitterdust, but wasn't bothering with power spells at all (she took Mass Reduce Person just because she thought people being small was funny), and now that we were higher level the deficiencies in the Beguiler and Dread Necromancer spell list were really starting to show. Now that we're 11, in really hurts. For example, my Dread Necromancer has Lesser Planar Binding and Planar Binding (the latter due to Versitile Spellcaster), but due to lacking Magic Circle and Dimensional Anchor all he can do is summon critters into Ghoul Glyphs and then raise them as skeletons (the Caster Level requirement on Bone Creatures, which is entirely DM fiat, keeps me from raising enemies and actually keeping their abilities). If I was a Sorcerer, this wouldn't be a problem at all, and I could fix our magic item deficiencies (it's a low magic campaign) by binding one of those nifty Dwarven Outsiders that can craft magic items for you. And I could Bind a Genie to get some Soarwood or Bronzewood and build potent fortifications (it's a campaign where that would help). There's all sorts of things I could do... but my spells just don't do that sort of thing, and the Beguiler wouldn't have been able to help. Meanwhile, the only reason the Sorcerer can't is she chose not to. Recently we got lucky and I managed to possess a Cleric 7, which finally let us play with Magic Circles for a day, and it was a huge power boost just to have that one day of access to the spells we wanted... which were just Magic Circle and Dimensional Anchor. Both of which the Sorcerer could have taken if she wanted.
So, while I love Dread Necromancers and Beguilers, their spell list just lacks the game changing spells that a Sorcerer would have had. And that's really the difference between Tier 3 and Tier 2... Tier 3 solves problems quite well as they come up, but Tier 2 can say "I want to change how we're playing. I think I'll do that." That doesn't necessarily mean wish loops or other such nonsense, but being able to do things like saying "what we really need is a base of operations. Give me a few days and two runestaffs, and I'll build an underground complex that we can use" or "we really need more magic items. I'll teleport us to the nearest known metropolis to buy them. If that doesn't work, I can Planar Bind someone who can make us the magic items we want, and call him again when the work is complete to purchase them." That sort of stuff takes the game off the tracks the DM had originally intended it to be on in ways that can be jarring at times. While the Beguiler can do things like "I glitterdust, now the encounter is basically solved" and the Dread Necromancer can do things like "I combine Fear Aura with Imperious Command and Never Outnumbered, thus solving the encounter without spending a spell" the Sorcerer can change the way the party operates on a more profound level, thus changing the campaign itself.
So to be clear, the difference between Tier 2 and Tier 3 is a bit more subtle than the one between Tier 1 and Tier 2. Non proactive players may find Tier 3 classes to be every bit as strong as Tier 2 classes, simply because they're not trying to pull the campaign off the rails a bit. But if they wanted to, they could, to whatever degree the DM allows before fiat and nerfs step in.
Really, on the basic comparison of the Dread Necromancer or Beguiler to the Sorcerer, it comes down to this: do you believe the Beguiler or Dread Necromancer have the best Wizard/Sorcerer spells of each level? And by best, I here mean the strongest spells that would be allowed in whatever campaign. In a very small subset of games that would be true... the DM would nerf anything stronger than Glitterdust or whatever. But in the vast majority of games, there would be stronger spells available. And the Sorcerer could take those, and the difference in power between those spells and what the Dread Necromancer or Beguiler has would be more than the class features of either of those classes, at least past the low-mid levels. That's something I'm seeing in stark relief in the game I'm in right now that has all three classes. I love my Fear Aura and my awesome familiar, but I'd kill for Magic Circle and Dimensional Anchor. And quite frankly, a Sorcerer with Animate Dead, Glitterdust, Cloudkill, Celerity, Dominate Monster, and Solid Fog, plus the Mother Cyst feat (which the Beguiler has) could do 90% of what the Beguiler and Dread Necromancer actually do these days, especially now that at level 11 the Sorcerer could create Necrosis Carnexes with Animate Dead. And they'd still have room for stuff like Magic Circle and Shrink Item and all that.
The other big issue of course is the massive gaps in the Dread Necromancer and Beguiler spell list. All the really big power spells have been removed (or, in the case of Planar Binding in the DN list and Time Stop on the Beguiler list, made far less strong. After all, the inability to get access to special abilities from bound creatures and the inability to bust out with Wizard level spells while in Time Stop really changes things). The only exceptions I'd say are Glitterdust and Colorspray on the Beguiler list, and maybe Animate Dead on the Dread Necromancer list (maybe Create Undead and Create Greater Undead, but DM fiat controls how strong those are). And then huge areas of spells have been entirely removed. Nearly all proactive spells are missing from the Beguiler list, while only some of the minion spells are on the Dread Necromancer list. Missing entirely are the construction spells (Magecraft, Fabricate, Wall of Stone, Move Earth, etc), the magical trap spells (Explosive Runes, the various glyphs), the permanent buff spells (Permanency and everything it can do), and most of the traditional arcanist utility spells (Teleport is a big one). Minion spells are nearly absent on the Beguiler list (one might count Dominate Monster and Charm Person) and limited on the Dread Necromancer list. Since proactive power is the kind of power that can really unbalance games quickly, especially if no one else in the party can do it, this matters a great deal when we're talking about intraparty balance. Really, the Beguiler and Dread Necromancer end up very much like the Crusader or Warblade... strong in their areas of expertise, and competent generally, but greatly lacking in many areas of the game such that they do need the rest of a party to help them out. The Crusader is incredible at keeping the party alive while still dishing it out in combat effectively and being able to handle Diplomacy when fighting isn't called for, plus he can smash through walls when that's needed, but can't teleport us around or help with our magic item problems. The Beguiler can locate enemies and disable large groups of enemies and use illusions to really mess with people, but he could never do anything on par with building castles in a few days. And so on.
Sorry if that was a bit too rambling. I mean, what do you feel really puts the Dread Necromancer and Beguiler on a level with someone who really can just get us access to magic items when we need them or build fortifications on the fly or contact gods for information when we need it? Because right now, that's what our party actually needs. If my Dread Necromancer was replaced with a decently optimized Crusader, the party dynamic would change but the overall power level would be pretty darn similar. If it were replaced with a Sorcerer/Mage of the Arcane Order or even just a Sorcerer who picked her spells based on what was needed instead of what was funny, we'd be playing a whole new ballgame. I'm seeing how badly we need a more proactive class right now and while the Factotum's abilities have really helped out, they're just not enough to really do something big (though she did grab Shivering Touch in the clutch, which was great. Take that, Dragon!).
JaronK