I can't believe that some vets would give out such terrible advice! Thankfully, most people I know are reasonably experienced cat owners and know better, but what about cat newbies? It's like a vet lottery - whether you get a responsible one or not
I'm shocked that so much is down to "chance". I suppose it just reinforces what I've always felt - find an experienced pet owner (for the type of pet in question) and go by recommendations.
Even without the kitten and wandering issues, I would still rather have cats "done" at 5 - 6 months (where possible - obviously dependent on how old the cat is when you get it, and it's history, in the case of rescues), for the simple fact that they recover so much faster when they are young. I'd hate to put an older cat through an op (however routine), unless ther was no other option (ie feral or stray cats adopted later in life).