I agreed to never breed Ava or Chai but one of the reasons I felt as comfortable as I did with both their breeders is that they BOTH advised against spaying either of them due to their size and the risks of the surgery not outweighing the benefits. This to me shows that they truly cared about the dogs and not protecting their lines at all costs.
I believe that many of the breeders on here would be able to trust their customers in this respect as well. Most yorkie owners are extremely responsible and can do what it takes to "prevent accidents". It is not that hard. Certainly worth the effort to avoid the pain, risk and expense of this major surgery.