I cycle-commute on my MTB because it's the only bike that can handle the +80 pounds I lug around in twin oversized pannier saddlebags. I tried the road bike once and it wasn't strong enough. Additionally, the MTB is a pretty comfortable ride with a more cushioned suspension front and there's no need to try and put the hammer down too hard. I take the long out-of-the way route that takes me on a bike-path through woods, fields, along a lake and river then feeds me off into the neighborhood to maneuver to work.

At 5:30am, it's dark and there can be path hazards in the winter (branches down; standing water). MTB is a far better choice. Does it make me look less cool commuting? You bet. Do I care? Not one bit.