I have an old 26" mtb and use it once in a while for commuting. Last Fall I had it set up as a pit bike for cross racing so it still has aggressive tires and egg beater pedals.

That old mtb is less than ideal for my commute (22 miles each way), but I still use it occasionally if i want the workout and have some extra time. There's also some good training to be had when I commute on that mtb, like sprinting between stop lights in town, and a little offroad action on a dirt path next to the paved MUP. A little variety in the commute breaks it up and may even help me next cross season improve my performance.