While it may be slower, riding a mountain bike on pavement boosts the safety-commuter confidence. I ride a Cannondale F7 to work which gives me a decent workout despite the short distance. Sidewalks are crap and potholes/froze heaves are abundant. Also, the winter sand/gravel is around so knobbies are a plus. Lastly, when it's raining, it's nice having disc brakes.