I was specifically considering Rogues and undead when I made this, because it came up so much for me (or in games I was in).
Consider the following. Imagine if you will that the DM has a story in mind for how his campaign will go. Imagine this story as a road meandering through the country side. A railroading DM might have a very narrow road, while a sandbox DM might have a very wide road, but basically the DM has a story that he intends his players to travel along. It goes from story element A to story element B. Some DMs build this road right before the gaming session, others build the entire road months in advance, but suffice to say there's a road of story that they intend the party to follow (though the party may wander back and forth across the road).
For the middle tier classes (T3s), the DM can pretty much stick with the road they wanted. The PCs are unlikely to go too far off the road, as they don't have abilities that really send them flying off in other directions, but they can have fun whereever the road takes them, as their classes are flexible enough for that.
As you get to the weaker tiers, the classes start being unable to follow the road, meaning the DM has to alter their road for the PCs. For example, I had a DM a while back complain to me that he hated DMing for Fighters, because sometimes his idea for the campaign that day was about intrigue or stealth or something, and he didn't want a fight... but he had to put in some combat, because if he didn't the Fighter player wouldn't have any fun and would have to stay at home in camp, as he would just be a liability otherwise. And so the adventure for that day included the whole party sneaking into a hostile town disguised and gathering information, while a random monster attacked the camp that the Fighter guarded. A similar situation happened to me when I was setting up places to go in a more sandboxy campaign... I set up four options. One was mercenary work, one was hunting down an evil necromancer, one was tomb raiding, and one was spying against evil dwarfs. I set this up without knowing what PCs were going to show up. When they did, I realized two of them used sneak attack as a primary attack... and thus they refused to go after the necromancer or the tombs. The result of stuff like this is that the DM has to move their road to play to the PCs strengths a lot more, even if the story in their head really doesn't fit with that.
As you get to the stronger tiers, PCs start to be able to jump right off the road. You wanted to do a ship based adventure as they cross the sea, but the Wizard decided the 20gp price for a ticket on a boat was too high and just teleported the group to the other continent. You wanted a low magic campaign and the Sorcerer just used Planar Binding to get a Midguard Dwarf, offering the pay the dwarf for custom magic items. You wanted a horror campaign and put the party in a haunted castle, and the Wizard just cast Plane Shift because the party wanted to go to the City of Brass and hang out there. At this point the PCs don't need a road... they can go whereever they want. Now you have to build walls around your road if you want them to follow it at all (usually in the form of house rules, nerfs, and gentleman's agreements to stop teleporting everywhere and ganking every dragon in existance with Shivering Touch after researching their locations with Divine Insight and Lore of the Gods... yeah). Or you, as a DM, learn to adapt REALLY fast to the player's actions. But that's very hard.
So that's actually part of the system as written. How much do you need to change your campaign plot line to let your PCs participate at all, and how much do you need to fortify your plot line against PCs just leaving it and going somewhere completely unexpected? As a DM, I feel you need to know what you're going to have to look out for, and how the classes you're allowing can force changes in your game. Part of this system is to allow you to see that in advance, as a lot of people will get blindsided by what some classes can do. Heck, a recent addition to our gaming group decided to make a game world where magic was failing and magic items were becoming unreliable... he never expected us to use Planar Binding to make a non magical Soarwood Zepplin (thanks, Djinns!) and sail around as steampunk skypirates in response! But that was the effect of just one Wizard in the party... one super genius with a lot of (admittedly fading) power in his hands.
JaronK