Hmmm. I have only been asked twice--both times on a two leg trip to Uganda when I had been very careful to get good aisle seats as it was a long and trying trip in coach. Both people had less than great seats and seemed a bit surprised that I said no ( what I actually said was sorry, no, I chose this seat because I wanted an aisle seat) . I was surprised that they would think I would trade for less. Both times it was not to sit with a friend or family member , but simply that I had better seats.
I guess my biggest reaction was surprise that they would even ask.