For Mages at least this goes beyond doing things differently but equally well.
Our third role has no use outside of 20-man raids. Or to put it another way; our third role has no use in the vast majority of the game content. Add to that that you will never see more than one Mage Support role in a raid.
Now imagine the QQ if that concept applied to any role belonging to another class.
Would I play a Mage Tank if that was our new role?
Probably not, Tanking the Prince in HK is enough for me.
But hey.... give us a Tank like Justicar with an AoE Heal that only costs 6 points into the soul that is instant cast and no CD and I'm all for it.
At a Role Level
I can accept that Ranged DPS is just another way of doing DPS. The question is can Clerics, Rogues and Warriors do the same.
Or will they continue to insist that melee DPS should always be better than Ranged DPS?