I have a WCS from a breeder who owns his granny, his mum, his uncle and his sister, he has field trialling lines from 3 generations back on both sides and a COI of 4%. To be fair that is low for cockers, the all-breed average is 10% and for workers I understand it's higher. His breeder selects stud dogs for her occasional litters based on desired temperament, good joints and overall health, and as part of overall health she considers genetic diversity is important. (She also doesn't breed from adolescents...or bitches older than 7...or back to back litters... and she knows the name and circumstances of every puppy she's ever bred... but those are all other issues.)
We know that high COI is bad for individuals and for a breed as a whole, it ultimately impacts things like fertility and litter size, longevity, prevalence of hereditary health conditions, and incidence of immune problems and some cancers. Maintaining genetic diversity is important.
The opposite ie line breeding, which means breeding two closely related individuals together (not the same as breeding to maintain your own line, which doesn't have to involve inbreeding, it's a choice) is what you do when you want to fix or keep a desired characteristic - for example, size. [Entirely coincidental I'm sure that LL was breeding very small, fast cockers which a lot people think are undesirably small...hmmm....] When you choose line breeding/high COI to get more of one particular desired characteristic, though, that is a choice to compromise other things. And you may get a higher number of less healthy individuals, or puppies who have other undesirable characteristics because you've put all the emphasis on getting your one top priority.
My boy's litter, with the low COI, are pretty diverse in size and personality, but they're all healthy, strong, well put together individuals with a good but not insane work drive who are a pleasure to live with.
It's a question of choices and risks and how you weigh them as a breeder and your personal priorities.