I had that discussion with a FA several years ago. On a crowded flight she wanted me to take my small carry on and put it below the seat in front of me. I politely explained that I was tall, had big size 12 feet and had already paid to check a bag that I could have justifiabley put into the overhead bins, thus already saving a large amount of space at my expense. Why, therefore, should somebody who paid to check his suitcase, needs the under seat room, give it up for somebody who has NOT paid to check his suitcase. She agreed with my logic.

Had they forced me to take the small bag out of the overhead bin, I would have done so. And, on later flights stopped paying the airline $25 for checking in my bag, and instead brought it on board thus taking up even more space.

Of course, the airlines luggage policy creates this goofball situation in the first place. It is a policy that pits customers against each other.