What's so bad about monoergism and monothelitism?

Two early heresies that intrigue me a bit are monoergism and [monothelitism](http://en.wikipedia.org/wiki/Monothelitism) . Both basically suggest that while Jesus was both human and divine, he only had one "energy" or one "will." Both were eventually called heresy by the church, but why? Does this this guard against the frankly worse heresy of Nestorianism?. What is so bad about these heresies? What practical effect does it have on one's perception of Jesus if we fall into this trap?