Every time this topic is brought up, people try to justify it by stating that "all breeds today started as cross breeds."
While that is true, that excuse is not applicable with the 'designer breeds' being bred now. Yorkies and other dogs were developed as breeds for a reason. They were specifically bred to perform specific tasks. The results of the breedings were carefully observed, health issues were identified and a coordinated group of breeders worked together to combine carefully kept records and results and achieve the same goal. Eventually a standard was set that the dogs were bred to.
That is not the case with this mixed dogs being bred today. They are being bred willy nilly to meet the demand that comes with the current fad. There is no set standard, no guidelines for what health tests need to be done, no comparison of records and results amongst multiple breeders to determine any issues coming up and no need to even start off breeding quality dogs.
There is no valid reason that can be given as to WHY are these dogs being created. The only realistic answer that can be offered is 'because there are people willing to pay for them,' and that, IMO, is not reason enough to breed them and play around taking a chance on what problems one may be creating.