I always work my horses pretty hard, so yes, I use them pretty much all the time; and especially on horses I am training...no sense in damaging someone else's horses legs!
If I am not planning on working a horse hard, I will just wrap the fronts, but if I am going to work them hard, I wrap all four. I have a front set of SMBs (gotta get rears), and just use polos on her hinds right now.
When I work a horse, it's for 2-3 hours at a time...the rest of the time they are unwrapped, so I don't believe that whole "you'll weaken the legs" business...if they are used 24/7, sure, but for the time they are worked? No, I don't think so.