I assumed that it was to keep level 80s from farming exp mobs in Northrend, and the nerf to dungeon exp was an unfortunate side effect. If they wanted to stop people from farming heroics for exp, then it doesn't make sense for them to allow daily quests to continue to give full exp. With the new zones crowded, farming WotLK dailies is a pretty easy way to gain exp and some extra gold. And with the nerfed xp from mobs, nearly all of your exp will go to guild leveling as well, and you will also be farming guild rep.