In my region of the world (Canada), there are “professional” QH breeders who overbreed every year, cull the young stock, and send scores of nice enough registered young QH to the slaughter plant regularly. Some make it into the “rescue” pipeline and are good bargains.
There are also feral horses and horses on native reserves running loose and breeding. So no end of grade horses around, of varying degrees of quality from basically QH to all kinds of spare parts pony and draft crosses (I grew up with one, wonderful horse, but even at 15 I knew I shouldn’t breed her!). Again, always going through the rescue pipeline too.
Then there are freebies or super cheap retirees from the local TB and STTB tracks. My point here being that even papers don’t guarantee a good life to surplus horses.
OP, if your horse wasn’t a pinto, would you still be thinking about breeding her? Is it her basic conformation you like, or is it the spots? If it’s mainly the spots, don’t.
I think one of the complications about breeding grade mares is that people tend not to get around to breeding their competition or using mares. This point came up on another thread where someone was asking about foals out of mares with competition records. The paradox is, that you use up the mare’s young years training, and then if she really is a good horse, you use up her middle years competing, and then suddenly you have a valuable 18 year old maiden mare that needs to step down, but no one is going to risk her first foal at that age. So people are more likely to use mares of good bloodlines that haven’t competed for breeding.
This means that if you did train up your grade mare and have a lot of fun with her, it might be hard to then find the down time to breed her.
It’s different with race horses because they retire so much earlier. And interestingly, there are a lot more OTTB geldings out there being ridden, than OTTB mares. The breeders must tend to hang onto the mares. Which would suggest that you can buy a nicer OTTB gelding than mare, too.