Righteous Fury has an interesting little piece up about off-specs-- you know them, they're the weirdos who play their class to do something other than its standard role in endgame raiding. Feral druids, DPS-adins, Fury warriors, Enhancement shammys, and (perhaps most of all) shadow priests. The convention says warriors spec prot, pallys heal and buff, and priests use +healing gear, but these guys (and girls) choose to buck the curve.
What he says rings true to me because the guild I play with has finally reached the point where we're getting serious about raiding. My GM hasn't asked anyone to respec yet, but he's put it out there that we want to beat these bosses and get the gear-- if you can't do the job given to you because you're specced for PVP instead of raiding, you've got to move out of the way for someone who can, or do what's best for the raid. And I'm right there with him. Holy priests are the best healers in the game, so most priests should be holy and healing. Protection warriors have the tanking tools, so if you're MT, you better be prot.
I'm not saying there's not room for off-specs-- there's a guy in my other guild who's such an awesome shadow priest (shout out to Gregorus on Thunderhorn-A!) that the whole guild has shadow priest alts just because they've seen what he can do. I main tanked for him in Strat, and when he told us early on that he "tanked by healing," he wasn't kidding-- the mobs all hit him way more then they hit me, and yet he kept us all healed and up with Vampiric Embrace and his +shadow gear. After seeing that, of course I had to have my own shadow priest, and when she hits 60, I already expect that I won't be able to raid with her under normal circumstances. No problem-- I'll melt faces in PVP with her, and raid with my prot warrior.
On the other hand, if I need a DPS for a raid, and a shadow priest can roll out as much damage as a rogue or a warlock, then we're talking. But from what I've seen, off-specs can do those things, just not as well. Gear to gear (without extremely expensive potions), can a feral druid tank as well as a warrior? Can a shadow priest out-dps a rogue? Because if not, I'm giving those spots to PVE specced raiders.