I have a 9 mile one-way urban commute in Atlanta, GA. It's mostly on-road, with a little time on bike paths.

Having ridden both a MTB and a road-bike, here's my $.02:

1) Speed
This was the main reason I switched to a road bike. I wanted to go faster. I don't think my time changed all that much, though. It was like 50 minutes, now it's down to 45. It could also be due to increased fitness.

2) Price
I only look at used bikes (craigslist) but from what I've seen, there are many, many MTBs for sale with a wide range of prices, while the road bike selection is smaller and the prices higher.

As weight goes down, price goes up. HOWEVER!! You will be carrying all sorts of extra weight anyway (rack, panniers, clothes, lights, etc.) so why spend tons of $$$ to get a light bike that you will then weigh down with all sorts of stuff?

3) Comfort and Carrying Capacity
Also high on my list for switching. Riding my MTB made me feel like I was reverse-sailing. The upright position gave me a constant feeling of fighting the wind. Even a slight headwind was awful. I believed that a road-bike would be better for that. It turned out to be somewhat true. I "feel" more aerodynamic.

Carrying capacity is tough for me to compare. My MTB did not have holes on the drop-outs for a rack, so I always carried a backpack. My road bike has holes in the drop-outs, but no braze-ons near the seat stays. I used the p-clips to hold the rack to the seat stays. My view: this is the most important thing for a commuter bike. A good, solidly-mounted rack with a set of large panniers is essential. I think this will affect the quality of the commute more than many other considerations.

4) Flexibility
An MTB that you already own is obviously the best bang for the buck. Other than that, I think it's perfectly acceptable to have a commute-only bicycle. Commuters tend to face much worse treatment than recreational bikes (at least as far as road-bikes are concerned).