I commute on PD-M530s and low end Shimano MTB shoes. As far as power transfer goes, I don't think there is any noticeable difference between those and my road bike shoes/pedals (also Shimano). The problem is that the MTB shoes are heavy. Not a big deal for the commute, but on a long road ride I would notice it. I bet if I had a lighter, high end MTB shoe that I wouldn't notice much difference between those and my road shoes.

I'm willing to bet that a high end MTB shoe would be fine on the road for the average Joe.