There is a VERY high percentage of older bitches that are prone to and die from Pyometra. As dogs age their immune system starts to fail and this is one reason they are prone to it. The same holds true to some extent for males with prostate and testicular cancer. In my 6+ years as a vet tech I saw this to be true and in my mind there is no contest to spay and neuter. It should be done when the animal is done breeding. The only drawback I see is that if you wait too long and the dog is too old there is a much higher risk for things to go wrong during surgery, as well as the healing process. Unfortunatly, spaying a breeding bitch does not eliminate the risk of breast tumors and cancer.
Debby is right the biggest advantage is that you eliminate the possibility of Pyometra. As for breast cancer - no. If you spay a female before she ever goes into season for the first time you eliminate almost 100% the possibility of breast cancer, once she goes into season that drops to 50% and past that there is no real difference.
Here in Grand Rapids one of the big medical research centers is doing a lot of work in the area of breast cancer as the link between human and canine breast cancer is close to identical. One of my girls - a rescue JRT - had breast tumors removed this spring and is involved in the research project.
A third vote for spaying an older female, or any adult bitch that's not going to be used for breeding--a closed pyometra, or an unnoticed open pyo, can kill a bitch, and no dog needs to be undergoing emergency surgery when she's already trying to battle an established uterine infection...and your checkbook won't like the costs of that emergency surgery much, either. Better to spay (or have just a hysterectomy done, leaving the ovaries behind) while the dog is healthy and avoid putting her through the infection.
And ditto on the mammary cancer benefit: as soon as the bitch goes through one heat cycle, you've lost your 100% prevention, and once she's had a second cycle, there's virtually no prevention from the spay.
There is no relationship between what type of diet a bitch is fed and whether or not she will develop a uterine infection after a heat cycle. Raw- or kibble-fed, the fact remains that every time a bitch goes through an estrus, the opportunity for bacteria to enter the reproductive tract is created and an infection can become established in one or both horns of the uterus.
As for the death under anesthesia, that is a risk taken with all general anesthesia, for humans and animals alike. Such deaths are not common in healthy dogs, however, especially with the newer anesthetic agents and the addition of monitoring equipment in the operating room that is comparable to that found in the OR of a human hospital. That said, there is sufficient anecdotal reporting to suggest that the Malinois, like greyhounds, may be more sensitive to general anesthesia, not in a small measure due to the low body fat ratio usually found in most specimens of the breed (you rarely see a fat Malinois or a fat greyhound).
While no vet ever wants to lose an animal due to adverse response to anesthesia, in dogs like this you still have to weigh the options: if this particular dog you refer to died on the table during a routine spay while she was perfectly healthy, what do you think her chances of making it out of the OR alive would have been if she'd needed an emergency spay in response to a closed pyometra that had rendered her systemically septicemic?
The decision is always up to the owner of the bitch as to whether the risks posed by general anesthesia and surgery are worth taking or not. I can find no good argument for leaving a senior bitch intact if she's not going to be intentionally bred that doesn't fail in the face of the risks posed by having to have an emergency surgery done on a dog that is in crisis condition due to a pyo...but to each his own.
I have heard of (but not read) recent studies that show that bitches who are not spayed until after 5+ years old may live several years longer than bitches spayed early in their lives.
That said, as long as the female is healthy, I think it's better to spay a an older female who will not be bred again. In particular, I think it's better to do the surgery while the dog is in good health and able to recover easily from the spay surgery (as opposed to having to worry about pyo or pregnancy in an older dog).
I lost a 4 year old during a spay due to anesthesia. She dropped like a stone the second the needle hit the vein. Til then she was perfectly healthy and happy. I also lost an 11 year old to pyo. I now always spay a bitch if she's not going to be bred again as long as she's healthy. If you've ever seen the size of a uterus in a big bitch with pyo, you know a pyo spay is major surgery and very risky. I once weighed a uterus we removed from a bullmastiff with pyo. It weighed 6 pounds and the horns were almost as big round as a man's arm. There is also a very real danger of nicking one of those horns spilling infection into the abdominal cavity - peritonitis.
Having dealt with breast cancer in 2 dogs, both dying as a result, if I ever get a female intended as a pet, you betcha she will be spayed before her first season. I've done that many times in the last 50 some years. I sold female puppies on contracts that they would be spayed before their first season and they were. No breast cancer, no skeletal issues, lived long healthy lives. I'd have to go with what I have seen work in my own dogs and for me the benefits of an early spay far outweigh the negative risks of waiting or not spaying at all. A dog spayed young might mature with more length of leg than she would have if not spayed, but so what? Preventing breast cancer trumps cosmetics any day.