I used to do 15-25 miles (one way) on an old Trek 930 (rigid MTB) with the smallest tires that would fit on the original rims. I made no other changes to it for commuting except adding a rack. It worked fine for gravel trails but I didn't do any real off-road on it.

Then some SOB pried the shell of my truck open and stole it.

When I have a larger home (studio apt) and have finished building up my LHT for touring I will be going on a hunt for another 930. After commuting on it, an old Raleigh Supercourse road bike and a Schwinn Speedster 3 speed I don't think you can beat a good rigid MTB for city commuting.