A few quick comments:
- I've had people who were seated beside me get up and go sit somewhere else. It's not that weird, and nobody gets offended.
- IMHO, you should NOT get up and go sit beside someone else you don't know (without a really good reason). That other person was about to enjoy a comfortable flight with some extra room, and you would be taking that away (basically saying your marginal increase in comfort is worth a significant decrease in their comfort - that's not right).
- If your goal is to sit next to the pretty girl, then you shouldn't be doing that.
For the longest while, prior to the carry-on-luggage madness of today, I would make a point of being the very last person to board the airplane. That way I could walk down the aisle and literally pick any seat I want, knowing that nobody else would show up to claim it.
...and by the way, it isn't the end of the world if you claim a seat and someone else shows up who actually has it assigned. That's happened to me too, and I just admit I was hoping to snag a better seat and surrender it to them. Nobody cares.