That would be even worse. If enemies are crowded together, casting Hero Turncoat on them would effect a fairly large number of Units, especially if Units can sit on the same tile together and go through one another like they could in DK1. (Or we'll have to deal with that annoying collision as we did in DK2)
Also, what happens when Keepers are fighting other Keepers? Thier Turncoat spell will become less effective compared to when they're fighting Heroes. Then there's also the problem of how the Hero Turncoat will become completely abusable/overpowered if the Heroes are fighting another group of Heroes. (You'll probably see this in Multiplayer)
So no, I don't think it is a good idea. It still doesn't change the fact that Turncoat is a rather abusable/overpowered spell because of the effect it has on Units.