Why do I use a MTB for my commute?
The three things you won't find on road bike: Disk brakes which are better for the wet weather, a cargo rack for panniers, and fenders. So really a MTB is more versitile than a road bike.
The wider tires also help. I run 26x1.75 that aren't knobbies but do have good thread pattern as the stream in the park frequently leaves a nice slick coating of mud on the path.
With that being said, my hardtail is my commuter/errand bike, I have a dualie with big knobbies for real mountain biking.
I do notice a large difference in commute time when I use my road bike. I average 12.6 to 13.1 mph on the MTB and 14.4 to 16.5 mph on the road bike. Typical commute time on the MTB is 1 hr 55 min (13 miles each way). Using the road bike takes about 10 minutes off that time.
In really bad weather my commute shortens to 6 miles each way as I use light rail for part of the commute.

just my $0.02