We are on the same page LB.
The %age of error free homes would be very low and this would fill up shelters.
She and I have different faiths in the ability of average people.
I think the 'average' pet owner is not capable of sequestering their male dog from getting loose and humping another person's dog for the duration of that dog's life. Dogs are tricky, people are busy, and frankly the consequence of it happening is not scary. Attach a 1000.00 fine to it and maybe it might change. But not in this environment.
In a similar way I don't think the 'average' pet owner can keep their own intact female dog away from males for the life of their dog.
Given my lack of faith in average pet owners, I personally will continue to advise anyone that asks to spay and neuter.
(In addition I'm also not sure of the strength of this vet's argument - have to do more research - Her conclusion is not the norm at this point. It's just an opinion)