as to why this is a problem

You didn't really elaborate as to why this is a problem. There is nothing wrong with a hybrid, for example, being able to fulfill all three roles. There is nothing wrong with a warlock using Affliction over Destruction for a certain fight.

That would be a varied game gold wow play experience and it's important. That is at the heart of the argument for dual spec. You're right about one thing, that argument also applies to tri- or quad- spec as well as it is just as valid.