Should only those dogs without any negative genes be used in breeding. Of course not!
All negative genes have not been identified, new ones are found every year. One estimate for humans is that each of us has at least 7 lethal genes. Fortunately, recessive mutations only cause problems if the individual is homozygous recessive for that gene pair.
The number of Cotons in the gene pool is relatively small. Eliminating all carriers would eliminate many good dogs. Through genetic screening, we can reduce the number of carriers in the gene pool and avoid breeding two carriers of the same recessive gene.
Mutations are a normal and planned function of the natural world. Diversity within a population will normally increase, and can have a positive effect on the overall health of the population.
Health screenings documents that a specific part of the dog was normal at the time of the exam. For example, the eyes are one of the most complex organs of animals. And things can go wrong at any age. Therefore dogs (and people) are urged to get annual eye exams.
Dogs who had healthy joints at the time of the x-ray, can suffer injuries. Like humans, hips and knees are especially vulnerable to injury.