What I was told is that in theory the Road-style pedal allows for more torque to be applied, and for the pedal to be a bit more lightweight. MTB pedals are more designed to shed dirt/mud, and to be durable - so they're heavier.
Again, this is what I was told, and I also know several people who use MTB SPD's on everything, including road bikes. I'm a huge fan of the MTB SPD's, personally, particulary the Shimano PD-M520's.