Welcome to Yorkie Talk. Here is a thread in the YT library about breeding to start with:
http://www.yorkietalk.com/forums/bre...t-breeder.html
Breeding is labor intensive (hands on for the human), expensive, and carries a lot of risk for the breeding bitch.
Potential benefits of spaying: avoiding accidental breeding and things above, and preventing pyrometra, a potentially deadly uterine infection.