Interesting discussion - I happen to have a nightmare story about a dog who wasn't spayed early.
I am an American, and perviously most of my dogs have been rescues and therefore been spayed/neutered before I ever took them home. However, I have spent the last year in the UK and have been shocked by how many intact dogs there are! Anyways, I began regularly doggie-sitting for a friend, when I realized one day that the 3y/o female dog smelled very bad ... like pus... and I realized she had some nasty discharge coming out of her vulva (this was at the the end of her season). The vets told us she had pyometra, a deadly infection of the womb.
What followed was nearly 6 months of antibiotics, steroids, hormones, and countless other meds to help her fight the infection to minimize risk during surgery. At one point the dog's blood platelets were so low they thought she would need a blood transfusion to survive the spay procedure, which would have cost around £2,000. It's been such a long, stressful journey, but finally this little dog has got her spay and is happy and healthy again. We could have saved poor bonnie a lot of discomfort, not to mention the huge amount of expenses in vet bills we racked up, had we got her spayed much sooner.
The thing is, we were lucky to have caught the infection early. Many dogs with pyometra do not have smelly discharge, and can go unnoticed until symptoms of lethargy at which point dog may have only days to live and needs emergency surgery (which involves extra risk from the removal of an infected/swollen organ).
Now that I am adopting a little girl husky pup, I am trying to decide when to spay. After this nightmare, I never want to risk another battle with this awful infection. I will have a chat with my vet once I bring her home, and decide then. I just wanted to share my story to caution others with female dogs.