I have to say I have mixed feelings on this subject. Personally I would never send a horse of mine to slaughter, even if the option were available. If I could not keep it, or find a home for it where I was certain it would be cared for, I would have it put down. Slaughtering a horse that I own would be inconceivable to me. However, there are many reasons a person may suddenly find themselves unable to care for their horse(s), and there are also many uninformed or careless owners who neglect their horses everyday, and there are certainly some owners that purposefully neglect their animals.
Personally, I see slaughter as a necessary evil considering the current conditions and the excess of horses. So many backyard breeders, so many ppl who own horse(s) just for the 'status' of owning them, and so many neglected and mistreated horses are out there that there is no way we could ever care for them all the way they deserve. I would rather see horses humanely put down, and yes, used for their meat than see them live longer and continue suffering.
Is there a better option? Requiring a license to own or breed horses, &/or having to pass certain test and meet certain requirements is an idea. Although many ppl would certainly object to that proposal. Or, as suggested previously by Lady Dreamer and others, think of ways to stop, or lessen the number of ill-bred horses being produced. Educational programs on reproduction. Gelding clinics. Things of that sort. Come up with ways to create more quality homes. Get new ppl involved in responsible horse ownership.
Or work on making tougher rules and regulations for slaughter plants. Better ways to humanely euthanize the horses, strict standards that must be met. Group(s) of objective inspectors to maintain watch over them. Until we eliminate the need for slaughter (too many horses, not enough owners, and too many irresponsible owners) the problem will not go away.