From what I've seen online it does seem like most of the motives are behavioral. There are several places where people have asked about the pros and cons and were basically told that a vasectomy is preferred since only that produces behavioral changes (opting to alter a pets personality to avoid the need to train him seems rather selfish and lazy to me, but that's just me). Since it would retain dogs in the mating population while removing them from the BREEDING population though vasectomies seem like a large net benefit. The procedure seems to be rather rare at present, so virtually all males mating with females are intact and likely to breed.