High level roulette will probably get more xp eventually. They stated that they didn't want people to use them to level too quickly and adding xp to these dungeons beyond the bonuses they currently give would have been game breaking. I know many people, that had these given xp would 5 or more classes at 60 by now. After alexander savage is released I suspect we'll start seeing more xp. This happened at launch too, and gradually they increased it.