I've seen this too often, Ness, and it breaks your heart. Yes, people do get carried away when foals are going for a fiver each and they think, oh, it's so tiny, it'll fit into the garden. Take the stallions off the moor, I say and leave the breeding of pure-bred Dartmoors to the few dedicated to the breed.
Years ago, someone had the stupid idea of putting a small thoroughbred stallion on the moor with the idea of breeding superior riding ponies. Most of the resulting foals, I believe, died in their first winter though you do still get an occasional pony who's a bit finer than the others.
As to the suckers the only good thing these days is that they have to be slaughtered before being taken abroad.