I ride a rigid lightweight Cannondale MTB with 26x1.5 slicks for commuting, because the wider tyres give me a lot more confidence on wet hills roads, than my road bike. It also has lights and mud guards. The main reason I ride the MTB instead of the road bike to work is that the MTB is less likely to get flats, especially when I get to the city. It is quiet a fast bike only 15% slower than my road bike.

If you buy a MTB for commuting, try and get one without suspension. It really isn't necessary for bitumen roads.

CHEERS.

Mark