This is probably for the best actually. I'm quite certain that classes where the main focus is on debuffs (Or party-wide buffs) can't be balanced effectively in an MMO. For the most part debuffs tend to fall into one of two categories: Either they aren't considered very useful, or they're so vastly required you can't party without one. This works fine in an off-line game, where a boss being utterly crippled by a timely status effect doesn't ruin anything, or where going with a suboptimal party member won't get folks yelling at you.

As for the other question of why abilities seem to be "switched off" for certain fights... imagine if you COULD stun any attack. I bet you it wouldn't add more flavor and dynamic combat. It would simply result in the "required" party setup being one where you have enough stunners to stun every attack that's remotely a problem. The debuffs that are turned off, typically sleep, stun, silence, knockback, and bind, are disabled because they are easy solutions to problems the boss poses. The other debuffs, like slow, DoTs, Eye for an Eye, Virus, I don't think I've run into fights where these are disabled.