The front page often is a bland pile of lowest common denominator crap. We couldn't even go a week without dumb political stories which are covered everywhere else (though charitably more because of miscommunication that it was meant to be an experiment and only a week long, still). Lots of people don't even look at it because they gave up, though I'm not exactly close to that point.
This is all beside the point that flagging is (no matter what it ought to be) an extra signal that has broader uses than merely spam/rule violating. I could also argue that "off topic", a use mentioned earlier and on the guidelines, is sufficiently broad and subjective that "something I think the HN community would be better off not discussing" fits "off topic". In any case the flagging mechanism is still there. The site does remove flagging privileges if you use it too often, so there is clearly a sense of how flags ought not to be used too often (or else you lose them) but that hardly influences why flags ought to be used.
This is all beside the point that flagging is (no matter what it ought to be) an extra signal that has broader uses than merely spam/rule violating. I could also argue that "off topic", a use mentioned earlier and on the guidelines, is sufficiently broad and subjective that "something I think the HN community would be better off not discussing" fits "off topic". In any case the flagging mechanism is still there. The site does remove flagging privileges if you use it too often, so there is clearly a sense of how flags ought not to be used too often (or else you lose them) but that hardly influences why flags ought to be used.