This is probably one of the most controversial topics at the moment in the horse industry. Though I highly support horse rescue operations, I also am for the slaughtering of horses.
Just as seen with dogs and cats, there is more horses out there than there are currently people giving homes for them. With the economy how it is now, less and less people are able to continue to financially support a horse, so they therefor must sell it.
So what exactly are we 'supposed' to do with the excess? As much as some people believe it is possible, horse rescues and people are not going to be able to provide a permanent home for all of these horses. Now, just because many horses would be slaughtered in the slaughterhouses, doesn't mean that every one of those horses is a 'bad' horse. For some reason or another, they just became one of the many 'unwanted' horses.
The problem is, is that in alot of cases many of the horses had a reason why they became unwanted. Though I'm completely for horse rescues, I also believe that it is critical that caution is taken when looking in to getting a rescue horse. You have to remember that these horses came from that 'unwanted' population, or even rescued from neglect, which can result in some sticky situations later on. You could have a horse that seems like the perfect angel, then something triggers that past experience, and lookout. Now not every horse was neglected or abused that ended up as an 'unwanted' horse, but since the rescuers more than often have no idea as to the horse's history, there is no 'for sure' way of knowing.
It's definitely a tough topic. There are many great rescue horses out there that end up being amazing, well loved horses. You just have to remember you are dealing with a 'rescue' horse. It's been saved from a bad situation, and because of that it's to be expected that the horse isn't going to be the horse of your dreams. Though it very well could be.
It's really one of those topics that you have look at both sides of the arguement. Each side has a good arguement, it's just finding that balance between what's best for the horse and what's best for the human. Each horse is unique and has different experiences, good an bad, which should be handled separately. As the owner of the horse, they should be able to make a smart decision on what would be the best future for that horse, with safety of future owners in mind. But, though that's what 'should' happen, there isn't really a way of making it happen.