I don’t see the problem in voluntarily changing spec from boss to boss. As for hybrid classes like shamans and druids and paladins; I don’t actually think them changing spec frequently works out unless they have gear for those spec.
Also lets factor in the pace that Wrath is highly likely to play at, being that each phase might last 2 months given the break neck speed they added to TBC. Its just a fact that Wrath is 5X easier than anything in TBC, excluding a few bosses.
In actual wrath we literally RAN and full speed from boss to boss AE’n everything down, this is in both heroic dungeons (super common practice) and also in raids also, AE them down was the way it was done, we even AE’d ICC down also.
So I don’t see why spec swapping between bosses is even going to be a thing considering how ridiculously easy wrath actually is…
Just my thoughts, and only there to make you consider some other things, I get what you’re saying just don’t think it really works out that way because of how silly weak Wrath is.