I really can't see how the type of cocker will make any difference and even if you had a litter that had agility 'expert' parents how would you guarantee the pups would be any good??
No, you can't guarantee it, but genetics
can play a part in the success of an agility dog; a genetic pre-disposition to learn in a particular way, and have the confidence with the equipment can make the difference between a really good dog, and an excellent dog
Consider how important the temperament of the sire and dam is in the decisions of a (reputable) breeder.......the same thing applies to other elements of a dogs psychological make-up, too
People have dogs for different reasons; if someone believes that they will not gain as much satisfaction from their pet unless they can compete in agility/show/flyball then it makes sense for them to seek a dog that in their opinion has the best possible chance of offering this....assuming that the dogs are bred ethically of course