I assume that you were on either a 767-300 or 777-200 with two seats on each side. Generally, once it looks like most of the paxs are onboard (when I am stuck back in Y) I will lay reading materials, Ipods, etc. on the aisle seat to discourage seat poachers. If a late arriving pax comes along, often a standby getting a seat at the last minute, I of course just move my stuff. I agree if that pax wanted 21B she should have requested 21B. This situation would different on a 737 or 757 with three seats where a person is stuck in the middle and now has the chance at a better seat but there is still one empty seat to share between two paxs.