I have a dedicated commuter bike. It's set up with fenders and racks and semi knobby tires for the gravel and bumpy roads on my 4.5 mile commute. I also don't mind locking it up somewhere. I save my nice bikes for other rides. In the summer I may ride my one of my road bikes to work just for fun sometimes.

I'm acquiring parts to build a back-up commuter also (just for fun); I found a frame I liked on ebay for $50, just ordered some clearance sale wheels for it on gNashbar for $70.
My touring bike is also a back up commuter bike, but that's not the point.