There are quite a number of good reasons to wear good quality riding boots, and the ones mentioned above are some of the best, hairy legs and all.
I got a pair of Prexport boots from NewEnough.com about a year and a half ago, and they really do make a difference. Besides providing more protection than your run-of-the-mill Nikes, boots with vents are cooler and offer a larger area of protection, your feet fit better on the pegs and they provide more and better "armor".
One thing I never thought about until I read it was the fact that because they are generally lighter than say, hiking boots, the don't act as pendulums when you have an unexpected get-off at 120MPH, and your feet are flailing the ground, breaking toes, ankles, and anything else that gets in the way.
That's also the reason you don't want steel toed boots........