My son has been going to winter baseball camp at Florida State and working as a catcher under Mike Martin Jr. for the last 4 years. He is widely respected as one of the best catching instructors in the NCAA today.
He always addresses the issue of knee savers at the camps and he is a big proponent of using them, and all of their guys wear them (worked for Buster Posey). He said that it keeps the catchers from getting too deep into a squat which is a less athletic position and can promote laziness.