Marge: Whatever happened to “please” and “thank-you?”
Homer: I think they were killed in one of those murder-suicide things.
Manners still count, partly for the obvious reasons but also for the impact they have on the confidence of children.
When kids display good manners they get lots of positive reactions and feedback from adults. It kicks off a virtuous reinforcement cycle.
Contrast this with the reactions kids get when they are sullen and disrespectful (and are then surprised that adults treat them like kids).
Parents who don’t instill manners do a disservice to their children.