I am helping by being incredibly vitriolic. I just find you're coming off the wrong assumptions.
Here's an example: Sublime Chord. The requirements are fairly easily met (if a little late, as usual) and regardless if you take it as a 2-level dip and then take another spellcasting advancement PrC or you take it the whole way, it adds an incredible amount of power to the Bard, who's otherwise a 'Sorcerer Lite', so to speak. Bards can actually stand on their own somewhat decently if you know what you're doing (up until about 10th level), and they offer a good number of options (some of which are exclusive to them, like Song of the White Raven, Metamagic Song, Lyric Spell, etc. al).
Now try placing the Virtuoso and the Sublime Chord side-by-side (and remember that SC essentially makes a normal bard a sorcerer in disguise, and sorcerers are tier 2 in the other system): do you really believe that Virtuoso deserves that spot? Do you truly think that, BY ITSELF, the PrC provides you the power to bend reality over a table and fuck it so hard it can't walk for weeks?
You gotta scrutinize EVERY PrC in this way, and this is hard to do, because PrCs can either be too awesome on their own, or kinda meh except for some shiny features (Mindbender's the best example of this. It's the only 1-level PrC, ever). If you start off on the wrong assumptions to begin with, you can't go through with it.
Frenzied Berserker is a more difficult call. All he can do is fight, but he does that really well, enough to take down just about anything he can reach with his sword in a single hit. You're right that he's easy to negate, but really - the type of person who can negate him can negate most things. You're right that he's a danger to everyone around him, but that's not really a power issue, and questions of party-friendliness go a bit beyond the scope of a rating system like this. I do realize this is a controversial class, but I haven't really followed the debate. All I know is that, in a straight-up physical fight, they dominate embarrassingly. I think I'm going to keep them them there, just because they're such a huge jump in raw power over a straight Barbarian. Yes it brings its own weaknesses, so it's a bit of a special case, but I do think it's earned a top spot.
I won't deny that Frenzied Berserker is one of the if not THE class with the biggest melee damage outputs in the game.
Sadly, melee damage output is incredibly suboptimal, as it's denied far too easily. In order for a Frenzied Berserker to be effective close to 100% of the time, he needs four things:
-At-will flight (preferrably natural)
-True seeing at will (because it beats one of the simplest ways to get him, which is through illusions)
-A way to affect incorporeal critters
-The Mage Slayer line of feats.
If he can have all those four, he
might be able to always be effective.
Maybe. Just consider what a single flight-capable adversary can do to him (possibly even besting him at melee combat, no less, using flyby attack) if he happens to be unable to keep up.
He can kick stuff really hard, but that's it. He's incredibly overspecialized, doing one thing extremely well to the exclusion of all others. That means he's tier 3, because he doesn't HAVE the potential to break the game unless it's based off melee encounters all the time.