It is a very good question and I am going to try to be as impartial as possible.
Nowadays, it's really difficult for a breeder to continue his activity alone.
Cash flow problems, non-recognition of the profession, hard market...
The co-breeding permit to solve all these problems : It finances a part of breeding, it makes people come to the breeding and communicate around the breedings and breeders.
Co-breeding and the sharing economy could reinvigorate all the horse's sector and pump some new blood ! :)