This rule is probably one of the most important rules governing our social universe (OK, here I think I'm mainly speaking for college students, but feel free to correct me if I'm wrong). If people were to stop obeying this rule, complete anarchy would ensue and plunge the world into a dark and apocalyptic frenzy where nobody would ever leave his seat to go to the fridge for another beer or to go to the bathroom for fear that when he got back his prime seat on the couch (the one that isn't covered in cigarette burns and doesn't smell like rotten beer, of course) has been taken and will not be given back.
The ever important shotgun rule also falls into this category.
Breaking these rules is probably about as bad as crossing the streams.