I commuted on a fully rigid Marin Pine Mountain for a while. I set it up with some riser bars, relatively smooth solid centerline tires, etc. Some aspects of this were great for commuting - the bulletproofness, pretty cushy ride on fat tires, very low top tube for throwing the bike around in an urban environment. Ultimately, though, I couldn't get rid of my need for speed and went back to a road bike for commuting.

But that is just me, MTB's obviously work great for a lot of people for commuting for the reasons listed above and many others.