Yeah, same goes for pvp as well. In vanilla you can go into a bg as a pve specced mage or warlock for example and still do decent because it isn't THAT competitive. But there is absolutely no way you are taking your pve mage spec or honestly pve any spec into arenas with a FEW FEW exceptions.
Honestly I wouldn't mind dual spec in tbc. I'm going resto druid to relive that experience again and although I wouldn't be TERRIBLE in pve with my pvp spec it's for sure a pain in the ass to participate in both