My main commuter is a mountain bike and I use it because it can withstand just about anything. It has lots of braze-ons and has carried 60+ pounds of groceries with ease. I ride it in rain, snow, and anything else I want because I don't care about what happens to it. Plus I can hop curbs, ride down stairs, or whatever else may surprise me. However, if the weather is nice I will often times ride one of my road bikes. If it's utterly nasty outside, the mtb always gets the nod.