Just discovered this thread. What a load of rubbish in some posts and a long established therapeutic treatment, especially for joints and tendons. I think it is also used for acute injuries and early treatment of osteoarthritis because it slows the development of arthritis in joints for example. As you point out in another post CS, one treated blood collection can provide about 50 treatments, so it's not horrendously expensive either. Even if a horse is being treated with other NSAIDs for arthritis, having ACS available means that treatment can replace those right up to raceday and after the required withdrawal time for those other anti-inflammatories. That's why it would be used during racing.
For the idiots who can't read, it is about as far from blood doping as taking an aspirin for a headache. There are no RBCs re-injected and the injections are into the affected joint or tendon (or other tissue), not the blood.
It is true there may be additives, but these are not drugs as someone suggested, and have zero steroidal type effects.
Never heard of it being used as a preventative unless you mean preventative in the sense of preventing or slowing the development of osteoarthritis, for example.
Load of crap and mountain out of molehill stuff.