But the other part is that it's hard to keep up on class changes for classes I don't play. I started raiding in vanilla on a priest, and when I switched to rogue, any and all priest changes that happened did so outside of my gaze. I simply wasn't paying attention to them. And with the sweeping changes to classes across the board with every expansion, I never could really pick up my priest again, not in the same intuitive way I played her in vanilla years.
I'm not the only one who has this issue, and in Mists of Pandaria, there's a new feature to help address that to a degree. The Core Abilities tab doesn't break down every spell in your character's spellbook -- but what it does do is point out the core abilities for the class and spec you happen to be playing.