Yes, yes, the age-old question has popped-up once again. And once again, I will give my 2-cents worth.
As an avid road cyclist, I shave my legs. I also up-keep shaving my legs through the winter so that I won't have to do a full operation in removing them when bicycle season starts again. Here are the most notable excuses/reasons for shaving legs:

1. They reduce the drag coefficient for time trials;
2. Smooth legs feel fast when you're pedalling;
3. When you crash and hit the pavement, less hair or your legs mean less resistance, therefore the size of the road rashes are minimized;
(then I wonder what happens when you fall on your face if you have a goatee or moustache)
4. Shaved legs enhances the feel of massages;
5. They look good.

Of course, the most valid statement of them all is number 5. I know that for me, this is my primary reason among others. I'm certain that all the serious roadies out there will tell people differently, and they will return all other responses you see here. The truth remains, that everyone who shaves their legs do it because they look good. Once you get your calves and quads all toned up, who wouldn't want to show them off?!