One of the things that contributed to my son's leg strength was the karate he took for 4-5yrs before becoming a pitcher.
I agree with all of you that leg strength and fast legs play 2 different scenario's in baseball. It's well known that good pitcher's have strong legs, thus, pitching longer and deeper inning's per game.
My son, however, is not the swiftest of feet on the team. He is currently doing some offseason training by doing resistance band training on his legs (and upper body). These bands increase these "fast twitch" muscles which are made up of thigh flexor muscles and abductor muscles, also including your glutes and hamstrings.
I ordered this workout program off the internet for about $30, which includes the pamphlet and resistance bands. If anyone is interested, I can get the website and post it. I don't have it with me at this time.