Often my only carry on is a shoulder bag that is very similar to a laptop bag. It always goes into the overhead. I don't mind rearranging it to accommodate a roll a board but I wont put it under my seat....I want my legroom.

If I choose to check a roll a board / suitcase to save overhead space (or guarantee my one small shoulder bag can go in the overhead) I'm not sure why someone who didn't want to check a roll a board is somehow entitled to that same space just because their bag is bigger.