Well, but that is not what happens. In retail you pretty much can tank/heal in the same gear as you DPS, but that doesn't make every havoc DH suddenly become Vengeance or Prot Paladin play holy. That did not happen in WotLK, that would not happen in TBC.
And that is a big one for PvP players too. I for one would love to do PvP when I am not doing PvE, but it just discourages large part of community because you have to respec every time you want to go into arena.