I do use a massage therapist for Bailey... not often, or on a regular basis, but when he seems to be a little sore somwhere or if I've up'd his work...
Someone said that horses don't *need* massages, they are fine without them, and for the weekend trailrider or pleasure rider you're probably right... however a performance horse, show horse, or a horse that is in consistent work (there is a difference between working your horse and just going for a ride) will get sore, and can absolutely benefit from a massage. The woman I use is reasonable I think ~ $50.
A horse in work is an athlete, which I think is easy to forget. Like any athlete to perform your best you need to feel your best. A sore muscle will result in a horse carrying himself differently to compensate, which may cause other areas to be sore or other problems to arrise... such as not bending in one direction, leaning in/falling out on a circle, lead changes... etc etc etc.... all sorts of problems can come up.
The everyday horse may not need to be massaged on a regular basis but I think just like all humans can benefit from one, so can all horses.... especially because we ask them to do so many things with their bodies... and with the added weight of a rider.
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."