Riding on a little mud or gravel on my road bike does not bother me - but I've been riding a long time, have a lot of mountain bike experience, etc. If it bothers you, then by all means by a commuter bike with wider tires and maybe mountain bars, etc. American Breezer makes GREAT COMMUTE BIKES - or buy a used mountain bike to ride to work - You do not need a road bike to ride 8 miles to and from work.

I have an old mountain bike that I usually commute on just because I like to ride it. I think that commuting is usually a more laid back type of riding than what I usually do so it is nice just to cruise into work - no pressure, no riding hard, just a nice way to get to work and back. I also use some a combinatin of our excellent FUTS trail system and roadways to get to work so a mountain bike is little better for this combination. I hate to ride a roadbike on trails.