While they are called mountain bike cleats. I don't thing that was the intention. Mountain bikers just favored them because of what they allow.

1) you can walk more normally in them because they allow more area for shoe tread.
2) they shed mud better or mud is less problematic for clipping in to the pedal. Don't know this for fact. Just what I read.
3) typically they have a sole that flexes slightly more. Since the shoe has tread to let you walk, then certainly you want some flex to help make it easier.

I only ride rode bikes, but I too only use SPD or moutain bike cleats. The bikes shops around here have equal amounts of both. Some of the shoes for SPD look more like something a road biker would want to wear than a mountain biker. IMO.