Interesting. I know that glutes and hamstrings are extremely strong muscles and it'd be a real shame if people didn't exploit this. Do you have a link to an article that explains why it's impossible or counter productive to use them?

EDIT: also, for some reason a lot of cyclists think that hamstrings can only be used when pulling up. If your knee is sufficiently far back, your hamstring and glutes kick in on the downstroke automatically. This is why I personally advocate sliding back and lowering the saddle slightly - it really helps me dig deep into those posterior muscles.