Actually, polo wraps protect a horse from splints, bucked shins, windpuffs, bruises...etc. Splints, windpuffs, and bucked shins cause temporary lameness, but can be seen as a blemish for the rest of a horse's life. Also, if wrapped right, polo wraps do provide support when they are hooked just behind the fetlock joint. They provide support of the flexor tendons. Also, no extremely good horseman will ever wrap the wraps tight enough as to bow a tendon. Every great hunter/jumper trainer and dressage trainer that I have ever seen uses them. I've wrapped hundreds of horses, and have never had one bow a tendon yet, and I'm not even worried about it because I know how to wrap...
Here is a list of all the soft tissue injuries that wraps may prevent...
http://www.umm.edu/orthopaedics/soft.htm
Also, it has come to my attention from a very knowledgable race tracker that wraps may actually slow down a horse a little bit, and it would make sense because the horse does not have quite as much flexibility in their fetlock joint. There is some slight tension and resistence at the joint from the wraps. Alot of the major trainers of the sport believe this (or so I've been told...but my source is reliable). Of course, I would never ever ever let that affect the way I bet a race, or me ever using wraps on my horses.
Also, a lot of racehorse trainers use wraps as a means of preventing burns on the bottom of a horse's fetlock joint when they are running in a race. This is especially true for horses who are a little coon footed (long in the pasterns), because these horses fetlock joints will touch the ground while they are running.
Okay, Im done now.