For me: What IS the extent.... not much at all. , although you have to take into account the fact that I am a naturally able minded student who keeps my own grades up.
What SHOULD be the extent? I know it sounds really third grade, but I think that parents should take the time to sit down with there kids and go over the homework, even if the kid doesnt need it. This provides support for work ethic, and also if they dont need the boost in work ethic, then it at least shows the kid that you actually care about what they do during the day.