It is quite effective blackmail on their part too, because it basically gives you the choice to go to their seat, or sit next to someone for the entire flight that is probably going be irritated. For me, it just depends on what the seats are. I'm 6'9 so I really don't like the bulkhead seat. On 737-700 I actually prefer 13A/F to any of the seats in Row 1, especially on a longer flight or red eye.

Basically the same way I decide whether to move for someone or not, assuming they tried the "plop" trick. If I'm on a red eye, or a long flight, and I'm going to sleep anyway, I will tell them to go back to their own seat because I'm going to sleep anyway and don't care if someone is stewing next to me while I sleep. On the other hand, I will usually swap if it is a shorter flight (SEA-OAK/SFO) because I'm going to try and get work done and it will distract me knowing the person next to me is nursing a grudge.