I think I have a right to put one bag that is rollaboard size or smaller in the overhead. If the overheads were full and someone politely asked me to please move my one small item from the overhead compartment and stow it under my seat, so that they would not have to gate check their rollaboard, I would probably do that -- not because I am obligated to do so, but just because I am a nice guy. But as a general matter, I don't think I should be "penalized" for bringing a small carry-on bag rather than a larger carry-on bag.