Quote Originally Posted by Carighan View Post
But then at what point is DPS also tanking or healing, tanks also healing and healers also tanking no longer acceptable, if tanks and healers also DPSing so readily is? Why is one direction of role-invasion acceptable, but the other two are not?
Because one direction isn't actually role-invasion in any meaningful sense.

In an encounter model where the goal is to get the boss's HP down to zero as quickly as possible, damage dealt per second is an unlimited need. The only way a healer or tank can take away from a DPS's ability to fulfill that need is to actually deal more damage per second than the DPS -- i.e., the point at which people would say, "Screw the standard comp, let's replace a DPS slot with an extra tank or healer because it's faster that way."

On the other hand, healing -- for the sake of argument, think "HP restoration" -- needed is a bounded quantity. An encounter deals only so much damage to the party. This need is supposed to be fulfilled by the healers, but because only so much is actually required, any other role that can restore HP encroaches on the healers. And, indeed, if you can find a way to fulfill that need for HP restoration without a healer, you drop the healer and replace them with a tank or DPS -- anyone who actually deals more damage per second.