[loramin]

It's very simple: in raids you want as much damage as possible at once, and in groups you want as much sustained damage as possible. It doesn't matter if you can 3 shot a mob in groups because (even with Clarity) you're going to need to med for awhile afterwards. Meanwhile the rogue couldn't 3-shot the mob, but he does roughly a third (to make my math simple) of each mob's life, and keeps doing it mob after mob.

So if you consider the twot over the course of five mobs (assuming it takes you about that long to regen your mana):

Wizard: 100%, 0%, 0%, 0%, 0% = 100%
Rogue: 33%, 33%, 33%, 33%, 33% = 165%

Now the exact numbers may vary: maybe you only need four mobs to replenish your mana, or maybe the rogue actually does 37%. But even if you crunch with real numbers you'll find the same thing: certain classes (eg. rogues) offer more damage over time than certain other classes (eg. wizards).

P.S. Just to be clear, I'm not saying Wizards don't have value in groups; I'm only saying that if you focus on sustained damage over time they can't compete with many other classes, just as other classes can't compete with them when it comes to focused "burn" damage.