Why?
Heroes 3 didn't have mixed classes, but Heroes 4 does.
Heroes 3 has hero specializations, but Heroes 4 doesn't.
One replaces the other.
If you were to add hero specializations, it would be overkill.
At least, that's what I think.
I agree, in terms of gameplay, it's quite replaceable. But in terms of the game's atmosphere, I don't think it's very good.