Put it this way - the Kennel Club has been in existence since the nineteenth century. Most of the "others" have been around 20 yrs max.
Yes the KC does need to put its house in order regarding puppy farm registrations and it is making progress but, as they say themselves, the system is based on trust and is not a guarantee of quality. Quality is pretty impossible to quantify anyway. However if you buy a KC reg'd Yorkshire Terrier and turns out to be brindle, shortcoated and as big as an Akita, you do have some comeback - if the breeder is found to be dishonest (easier to prove these days with DNA), the KC can ban the breeder and you can sue the breeder and get your money back. If the same happens with another registry, you have no comeback.
A KC reg. dog will have a traceable pedigree, going back many decades or at least back as far as when the breed was 1st introduced to the country - I can trace all my dogs ancestry back to the 1940s when they came to the UK and from that further back from their country of origin.
If I had bought from another registry, I could go back to - wow- 1988??
Actually, there are other registries - like the ISDS and some old working breeds who have their own that are worth something.
Let's face it though, the only reason most breeders use "other" registries is because they are not eligible for KC Reg. At least the KC won't accept registrations from b1tches who have had litter after litter, are too young or too old to be bred from and their registrations details can be easily verified.
It would be interesting to know how many dogs registered with others carry out the recommended health checks on their dogs in comparison with KC breeders - virtually none I would guess.
You'll notice I don't mention any other registrations by name! They do keep an eye on these forums and say something derogatory about them and you are likely to get a very unpleasant e-mail!