I rode an MTB as a bike messenger with slicks in 1997 and 1998 because I had it in my basement (it was my old MTB before I replaced it). When I stopped being a messenger, I kept it for commuting because it did the job well until 2004, when it became my winter bike. I bought another MTB for commuting for robustness, parts interchangeability and because I liked the fit. I thought I could just change the fork on the new bike from a suspension to a rigid fork, but found out I couldn't because the angles would be too messed up. I also found out that I did not necessarely need the robustness of an MTB. I started to regret the buy a couple of years later when my commutes became longer.

In essence, unless you really need it, an MTB is overkill for commuting; an hybrid, touring or cyclocross bike is much better suited for the job. That's why I bought a cyclocross bike yesterday.