Two early heresies that intrigue me a bit are monoergism and 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?
I was trying to explain these two Christological concepts to an unsaved friend of mine, and he said, "Oh, so you're talking about the opposite of demon-possession - that Jesus the man was simply possessed by God."
Stripping that analogy down to its essentials, I think he may be on to something. If Jesus only had one "energy" or one will, then Jesus is essentially a docetist God, only appearing to be man. If his humanity is scooped out and overridden by God's indwelling, then he can't really identify with humanity. To hold to a monothelitist idea is to say that God only "appeared" to be human - but a lobotomized human isn't really all that human.
As such, to hold to this heresy would be to make a mockery of the incarnation. God may have taken on flesh and bone, but not mind and spirit.
And, if he didn't, then he didn't know jack about every Tom. Dick, and Harry out there...