This is dumb. Yes, Pain and Fury are effectively the same resource, but the name Pain for Vengeance's resource is an evocative and effective description, which is exactly what the name of a resource is supposed to do -- all things equal, you might as well just call them both "Energy", or call a paladin's resource "Combo Points" just like a rogue.
This is homogenization for the sake of it. It has nothing to do with the real problem of specs being too different, which is about players switching specs and getting a completely different loadout of abilities that they don't know how to use.