To me the only difference apart from potential enforcement, is the prioritisation of the people who have luggage in the hold and thus no actual NEED for overhead space (though perhaps desire to have some for speed of exit), from the hunger/angry cantpaymorethanthebareminimumcramitinCRAMIT lot...

That and Bronze separated out from the rest. Probably the second biggest contribution to the size of the queue after nobody giving a crap about who is in it...