I have 7 bikes that I can use for commuting. There are a couple of bikes that I hardly ever commute on but I have used them in the past. I also have a couple that I use most of the time but mostly I just ride what strikes my fancy.
I don't go the route that most people do on mountain bikes...I have 3. Each one is outfitted with knobby tires and, if I ride one to work, I look for places to use them. If I want to go fast and/or efficiently, I have road bikes for that. Or I just push harder.