I think most of us do what we can to rearrange the overhead bins first, before asking people to stow things at their feet. And yeah, I suppose it does nothing to deter people from bringing on too many or too-large bags. But understand that being the bag-size police isn't really what the FA's do...we don't have bag sizers on board, and our job at that point is to get all the boarding business taken care of, close the door and go. Gate checking bags can be a little time-consuming at times. It's really up to the ground staff to catch problems before they're taken onto the plane.