With females (at least older females) I am pretty sure the consensus is that the threat of pyometra makes it better to fix her. The risk becomes quite high in certain breeds from what I have read.
All of pets before this have been fixed and none have been fat, mentally deficient or suffered from any problems such as incontinence.
Our youngest female was born with hip dysplasia even out of tested parents and I have never heard of a link between dysplasia and spaying. Dysplasia simply means the hip ball doesn't fit into the socket due to abnormalities in the socket. Did the vets explain how surgery caused it?
Honestly for the majority of pet owners, I think the benefits of spaying outweighs the possible risks. It seems the longer Joe Public has an unspayed animal, the more chance that animal has of being bred.