This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
I've only encountered one case of a vet recommending not to spay a kitten because it was FeLV+. Unfortunately, going into repeated heat cycles was very stressful on her, and that owner believes it caused her to suffer ill effects of the disease sooner than she may have otherwise (of course there is no proof to this). It is true that there is a small added risk to anesthetizing an FeLV+ cat, but that risk will well worth the alternatives. Especially since a whole cat will have a very strong desire to roam, thus further spreading the disease. In my personal experience, my vet did a very careful screening to make sure my FeLV+ cat could handle the anesthesia before operating. It really should not be much of a problem, but always make sure if your cat needs surgery that you remind the vet and staff that your cat is FeLV+.