Their goal is to have all dps classes have similar raid utility. Priests' VE is a small raid healing cooldown, similar to that of mages new amplify magic. They work a bit differently, but they still serve the same basic purpose. Their goal is to also have all dps do similar damage. On live, shadow is indeed one of the weaker single target dps. But if they tune it right, then in WoD mages and shadow priest will be about equal, perhaps one doing better in certain situations, but as a whole balancing out. Their next beta build will include the start of their tuning passes, so if you see mages being ahead of shadow there, then you have a reason to complain.