Personally, I don't see anything wrong with leaving a dog intact, as long as the owner is responsible and precautionary (ie able to be 100% sure that the dog is not capable of roaming or mating).
Re cancers; Well, this is unfortunatly a common misconception. Testicular cancer is very rare and very easily cured by neutering. Cutting off the hormones greatly increases the risks of more deadly and inoperable cancers (bone cancer, hemangiosarcoma, etc).
Re temperament issues; Well, this tends to be more formed on opinion, if you ask me. Most reports find signifigantly more behavioural issues in altered dogs; However, this isn't to say that the issues would not have been there if they had been left unaltered. It HAS been shown that cutting off the hormones does affect the part of the brain that affects bite inhibition (causing the animal to have a higher tendency towards biting), but again, one doesn't really know how much higher the risk of biting is, and how one can weigh the risks of that vs dog aggression, marking, etc.
In my line of work, I do tend to find that the unneutered males for the most part are jerks -- they bite, they pee, they act like little *ssholes. However, these dogs are typically owned by people who only bring their dogs in on occasion, leading me to believe that the dogs are only unaltered because the owners don't want to spend money on them. If they won't bring them in regularily for grooming to the point where I'm completely shaving off a horribly disgusting matted dog who is uncomfortable and probably thinks I deserve to be bit for torturing it, then I can probably assume that they also don't want to spend the money on neutering it, or on training classes to fix the unwanted behaviours for that matter. On the other hand, my clients who aren't neutered who come in for regular grooming tend to be lovely little dogs; They are in the minority, because most of the males I groom are neutered, and most of the unneutered dogs are not regulars (ie they only come in once or twice a year).
I tend to start seeing males testing the waters around 14-15 months of age (in my breed). This isn't to say they wouldn't do it if they were neutered -- obviously one has no idea. In males, I have seen no difference in temperament after the male has been neutered (in other words I'd tend to agree with your vet, that neutering a dog who has developed hormonal behaviours will not make the dog any better after the hormones are cut off -- they have to be cut off before the hormones have already started to take affect on the temperament -- hence why we see some late-neutered studs who will still try to breed females). I HAVE seen some signifigant changes in FEMALES who were spayed after showing bad hormonal behaviours (dog aggression, etc), but alot of the changes were unfortunatly not ones that the owner wanted.

So, hormone removal can be a really touchy subject. On one hand, we want what is best for the dog population. On the other hand, we want what is best for the health of the animal in question. For a win-win, I would suggest a simple visectomy. The dog is unable to breed, but retains the hormones.
In most parts of Europe, this has always been the primary way of altering; Males recieve a visectomy, females have the uterus removed but keep their ovaries to continue hormone production. Dogs in most places in Europe are also allowed to nearly anywhere; Buildings, public businesses, etc. And yet we hear of little problems over there, as opposed to problems over here. So one can not convince me that leaving the hormones intact will cause major issues, when there is nearly an entire continent of hormone-intact dogs that go anywhere and everywhere.

Does Joe Blow Pet Owner need to know about the cancer misconceptions etc? No, because Joe Blow Pet Owner should spay/neuter their pet, regardless of how the procedure is done and what affect it has on hormones. However there ARE some researched, responsible people, who are capable of keeping intact pets. What I DO think more people should know about are the other options available for spaying/neutering and leaving the hormones intact. It's been done in Europe forever, it's been done in Canada for many many years, and is now starting to catch on in the USA. I would suspect that vets who specialise in reproduction are likely the ones to go to for this matter. In my own experience, too, alot of these vets coincedentally come from Europe.