I have two bikes, a "commuter" bike and a road bike. Like hikelite says, my commuter bike is more comfortable (I can ride it in my regular work clothes, if it isn't too hot out). I also commute with a lot of stuff (I'm a hs teacher and a graduate student, so bike with a lot of papers/books), so I've added pannier racks, etc. to my commuter bike and don't have to weigh my road bike down....

And, finally, I leave my commuter bike unattended for long periods during the day, and it would be less devastating to have it stolen than my road bike.