The only way a mixed breed has a better chance health wise is if it has lots of breeds mixed in. For example, our Sandy was a stray and has some shepherd, maybe some husky, maybe some lab, maybe some of this and some of that, and God only knows what else mixed in!
In this case, I wouldn't have her any other way, she is a BEAUTIFUL dog and very healthy too. I wouldn't think there would be much benefit to only having two breeds mixed together. Like others have said, you just open yourself up to both breeds' problems.
But, when you know you want a certain kind of dog, then I would definitely look into the pure bred ones....then, just make sure you do the best you can with a good breeder so you get a healthy one. And, like other posters said, you can really educate yourself on Yorkie health and care and do the best you can to take good care of them.
Good luck!!