Great links! Mind if I send my clients to that? Saves me from re-saying all that.
Usually it is my male clients that do not want to neuter their dogs and the main reason is that they are putting themselves in their pet's shoes. One thought that I use to help convince these reluctant male parents is:
If you are not going to let him breed, and he can smell every female in heat in a two mile radius - is it fair to leave him un-neutered? If he is never going to be able to do anything about his umm... urges; isn't he better not to have to worry about it?
When you compare the medical, behavioral, (and I think psychological) benefits to being neutered compared to the risks of anesthesia (minimal) and the disadvantages of neutering (mainly just needing to watch his weight) I think the answer supporting neutering is clear.