Recently on a flight a passenger took the empty seat beside me. He had an assigned seat that he left behind. If by luck of the draw I had an empty seat (true not paid for), then it seems to me that as a beneficiary of said luck that I have inherited certain rights. If the other guy had stayed in his OWN seat, I would have had the enjoyment of more space. His moving AFFECTED me. The only reason I point this out is because while my situation was benign, I know that sometimes these little irritations or frictions on flights escalate into real on board conflicts (fights), and while I am describing out a pretty subtle point here, I think that it is better for the flight crew to mediate between passengers using preventive practices (etiquette, courtesies, “rules” etc.) rather than letting passengers resolve them themselves, in those cases where we are dealing with seat assignments at least. – Trevor
I’m going to tell you what 90% of the flight attendants I know would say. You paid for a seat. One seat. Not two seats. Not an entire row. Just a single seat. So if a passenger wants to switch seats, that’s okay. The passenger is allowed to sit in “your” row. While at my airline passengers are free to move to any open seat available in their ticketed cabin, other airlines (regional carriers dealing with weight and balance issues and airlines who charge extra for certain seats in the same cabin), require passengers to ask a flight attendant before swapping seats. If the flight attendant says it’s okay, it’s okay, the passenger can move.
Just because you were lucky enough to to score an entire row to yourself does not mean you have “inherited certain rights.” Oh sure it’s annoying when someone who already has a seat invades your space, but imagine you are the one stuck in an undesirable seat and there are two open seats in the row behind you, wouldn’t you move? Should a passenger have to suffer just because someone else is the “beneficiary of said luck” when there is plenty of room for both passengers to stretch out and relax?
In the future, if you’d rather not sit next to anyone, try making your row a little less appealing. The most popular seat on the airplane is the aisle seat. Take it! Otherwise someone will plop down beside you. Then, after takeoff, spread out. Pull the tray table down and place something on top of it. Put a bag, coat, or book in the seat beside you. Pretend to sleep. Not many people are ballsy enough to wake a sleeping passenger. Try traveling with a packet of Kleenex. No one wants to sit next to the sick guy. Or better yet, travel with a child. Works for me. Passengers avoid kids like the plague. That said, if someone still wants to sit in your row, they can. So be prepared to move your things out of the way….
Heather Poole 
When I purchase a ticket to fly somewhere, I expect I will be seated next to someone, I have bought one seat and that is it.
Totally agree, you pay for a seat, you get A seat. I’m sick of breaking up fights between pax where one thinks just because a row is empty they can take it.
For example, pax moves to empty row, we need to lie a pax down who is on oxygen and needs medical attention. I explain to the pax he needs to move back to his seat and he kicks up a fuss! Sorry buddy, you paid for A seat, not three seats. And if I need the seat you moved to, well, sorry but the sick guy takes priority, and the airline will side with me if you complain about it. “Seat assignments are not guaranteed in the case of operational or safety requirements”