I gave up a window seat in J for another passenger checking in ahead of me at the CW check-in line at MIA a few months ago. The passenger was irate that he did not get a window seat, and in the interest of making sure I checked in before the deadline (I was late arriving at the airport) I offered my seat. The check-in staff were NOT pleased to swap seats, and actually tried to dissuade me from giving up my seat, but I insisted. Whilst waiting at the gate the Station Manager came over to me (I am a regular traveller on this route) and asked to see by boarding pass and giving me another, telling me that I would "be more comfortable in my new seat" which was in F!
This is not the first time I have been rewarded for giving up my seat; it pays to be nice sometimes (although to the OP I agree that you were well within your rights to decline, but be prepared for a grumpy travelling companion!).