I used to commute on my mountain bike because that was the only one I had. But, I switch to my CF bike on fair weather days because it is faster and the ride is less tiring. The mountain bike still sees commute actions when the weather is unstable or I need to haul stuffs. I do park my bike in the office so theft is not a concern.

Some people regard using CF for commuting is a waste, but I think it is worth it if it brings extra pleasure to your ride. A fun mile as a fun mile regardless of the type of ride. My commute is relatedly long and fast though (17.5 miles one way up to 19 mph average).