Without wishing to offend you, gnasher, it doesn't matter what you're breeding the dog for, basic genetic principles hold.
If you inbreed, you increase the risk (note the terminology: you don't guarantee) that harmful recessive gene characteristics will show up in your dogs. As has been stated elsewhere, these animals will be culled out, so it looks to an outsider that there isn't a problem.
Scotty said 'Captain, ye cannae change the Laws of Physics!'
Sadly, it's the same for genetics, no matter what breeders choose to believe.
'Deliverence' anyone?