First of all, breeding top-line horses needs to be continued. Breeding horses with excellent dispositions, conformation, etc,is totally fine. I also think it is fine to breed to get a horse you want. **However, I don't agree with ten babies from a down hill horse, but if you want a foal out of the mare you love to death, so be it.
Rescues aren't for everybody. It takes a lot of time and commitment. And, as EastoWest (I think) pointed out, you usually have NO idea of the bloodlines or anything like that.
I volunteered at my local humane society. All of my cats and dogs have been from a shelter, or strays. And they are all fixed. I have had two rescues horses. But, I must say, my appaloosa now, that has been bred for good qualities is the best horse I have ever owned.
Also, I am pro-slaughter. It's a fact of life, if there wasn't slaughter, there would be too many horses and not enough room. It also helps keep the horse industry alive. Think about it, all the horses that are starving to death, slow and painful, if they go to slaughter, quick stunning and death.
So, yes, rescue horses and horses from auctions are all out there, but not everyone has the expirence or the 'want to' to do that. I definitely don't recommend it. It's hard! My last horse I rescued, I had for 3 years! I worked with her, and by the third year, I was finally able to catch her with a halter and lead without her running off! I had no idea of her breed, bloodlines, anything. The poor thing ended up getting kicked and died that same year, but it was a slow, long progress, very frustrating at times. I have never dealt with a nurse mare foal or anything like that, but I imagine it would be the same.