I agree with the other posts I just had a couple things to add.

It can be deadly to your dog if she were to accidently get pregnant by a larger dog. She wouldn't be able to deliver the puppies. Even a c-section won't gurantee live puppies, they may be so large that the mother can't carry them full term.

Even if you do decide to breed (which shouldn't be done unless your dog is a PERFECT example of the breed & you are ONLY breedeing to better the breed) but you still take the chances of an emergance c-section, still-born puppies, the mother rejects the puppies & you are forced to care for them, the list can go on & on.

An unspayed female, especially an older unspayed femal has a greater risk of Pyometra, a eadly uterous infection. Suregery must be done ASAP or you risk your dogs life.

And of course the already mentioned health probelms associated with cancers & the like.