This will be a pretty radical move. Not that I don't agree the main PvM configuration is Hero + a lot of hunters, but this doesn't have to do so much with balance as with making the game a little harder. And, even if people had to choose mercs of different classes, finally a new trend (common for almost all players) would set in, but the game would be harder.
So, your suggestion has its Pros&Cons, but I am not sure the first are more than the latter.