I'm very much a we-live-in-a-society, follow the rules kind of guy, but if I checked a bag and only have my backpack in the cabin, you bet your ass I'm going to try and find a place for it in the overhead instead of cluttering up where I want to put my feet. The flight attendants can go scold the passenger with the oversized roller + backpack + 20 liter "purse" instead.

Yes, the logical rule would be 1 bag in the overhead per person. If they enforced carry-on sizes strictly and charged less for checked luggage the problem would probably go away.