I do commute on a MTB, it has a sturdy feel (as the road to work has quite a number of bumps and the like), but it also feels mushy, heavy (not that I mind the extra leg work), and strong headwinds can be quite annoying. I jump up and down quite a number of sidewalks along the route so I'm guessing a road bike would not be the best for me anyways..

That said, a couple months ago an expensive-looking road bike passed by my side on the commute, guy looked at me and mumbled 'ah damn bumpy roads'.. Put a smile to my face haha