I shave my legs for two reasons -- however I fully admit that the primary reason is for vanity. I work hard for these legs and damn if I'm going to let a thick layer of fur conceal the muscle definition that I've gained over the years. Otherwise I look like like every other schlub out there. It was also sort of sickening to look down after a long ride and see hundreds of little hairs sticking out of my shorts. Looking like a lycra/spandex clad chia-pet just didn't float my boat, so off came the hair.
The secondary reason, for me at least, is that I got really sick of riding along only to feel the distinctly annoying pain of a hair or three being yanked out at the roots (or worse yet, repeatedly tugged at) by my shorts. The happened more often than you might think, and it can really snap you out of your "zone".
A nice tertiary benefit is that I use less than half the amount of sunscreen to cover my hairless legs than I had to use when fully furred. Good sunscreen is expensive, so now I also have a financial motivator. SCORE!
Oh, and I shave all the way up to the pelvis/leg joint. No use shaving only to have ridiculous looking "hair shorts" after the gear comes off.