Some on Equine-L have heard this one before, but here goes...
I had a young and nutty Arabian (Cooper) who pestered my mare (Sabrina) to the point she kicked him in the shoulder (she's heavy, strong, mean to some horses, and shod all around). He went 3-legged lame from an abcess. Vet and I dealt with that, but once we stopped the bute, cold hosing, and antibiotics, he went right back to being 3-legged lame (hopping along in a very pathetic way).
Vet was very bummed (he has a soft spot for Cooper) and said it might be "sweeney" - damage to the nerve in the shoulder, that the bute had probably been masking it, and he might never be sound again. The symptoms were consistent - he was even losing muscle mass in his shoulder (probably due to favoring it, not sweeney). Vet was not optimistic, but gave him a shot of antibiotics and told me to start him again on the bute at dinnertime. But by dinnertime Cooper was going on all fours without a limp. I never did give him any more bute, just the antibiotics. It seems we hadn't quite cleared up the infection with the first round. After the second course he was just fine, and has never had any problems with that shoulder since.
My point is, it's possible for a vet could be completely mistaken - not that vets are idiots, but they are human. Before you give up on a horse, consider the problem could be something your vet just hasn't thought of, or hasn't seen before. Be sure to check out alternatives - could there be some infection or inflamation in the joint? Could it be your mare threw her back out of whack and needs a chiropractor? I think my vet really just started the antibiotics again to humor me (and to put off the other option), 'cause I thought it must be the infection coming back, and they couldn't hurt anyway - he really didn't expect it to get any better. (Although he was delighted the next morning when I called to hear that it had worked.)