I was recently on a TA flight from FRA to IAD and I had the isle bulkhead seat. Probably about 40 minutes into the flight and after the first round of drink services a guy came up and asked if I minded if he "took that seat" and pointed to the middle seat. I wasn't in the mood for confrontation so I moved my things off the seat, and he sat down.

The FA had already done drink service once so they likely knew he didn't pay for the seat. Although I suppose he could have been in the bathroom. I am sure they see it a lot and probably don't like dealing with it.

To me its not so much of a big deal. Its nice to have the open middle on a 9 hour flight, but nothing worth a confrontation to me I guess. It seems sometimes half these seats are open on takeoff, and if everyone got in the habit of "squatting" them, it could become a mad dash when the seat belt sign turns off. Or even as simple as keeping an eye on the mobile app and taking one before takeoff.