@Janneke:
And about the 'freak of nature' that Vickilb mentioned.. I saw a Bulldog today that died because.. well.. because he was a Bulldog: His lack of nose gave him a palatum molle that was too long.. He just couldn't breath and died.. He came all the way from Japan, because he was such a beautiful example of the breed..
I agree Janneke, the original standards of many of the Mastiff breeds have been altered to please owners and ring judges. Look at some of the older pics of Bulldog champions, and compare them with what wins in the ring today. They look totally different.
Some breeders bred a better dog, others, a "prettier" or "showier" dog, not necessarily a healthier dog. The tastes of the 'consumer' are most certainly influenced by what they see on TV with all the televised dog shows. They want a bullie with the most snubbed nose possible, or a GSD with the lowest back hips. They continue to breed because people continue to buy. In some cases, they continue to breed, not because it's whats best for the breed, but what's currently in vogue in the show ring.
When it comes to designer dogs, they are bred here in the US to sell to the consumer with little regard to what's best for the breeds involved and with little or no testing at all in regards to health issues.
They're like the soup of the day here. If you have the ingredients, throw them together and sell to the highest bidder. When your palate changes, get rid of the old ingredients and concoct a new soup.