They do have some medical benefit. I injured my elbow last fall doing overhead triceps extensions with just over 200 lbs. After a couple of weeks of really bad stiffness and more pain than I like, I was bitching to the physical therapist at the hospital where I work. She hooked me up to the EMS machine and shocked the crap out of my elbow every other day for two weeks. After each "treatment" I noticed decreased stiffness and soreness, especially on extension. According to that therapist, the key isn't how hard you can shock the muscle, but developing a pulsatile curve that helps decrease inflammation through the tendons. Also, she says they are worthless for "building muscle".
JK