I was on a lightly loaded flight and was given the window seat F, there was someone in the middle seat E and the aisle seat D was empty.
The pilot announced "for reasons of balance you must stay in your allocated seat".
I looked at other rows and many of them had passengers in A+C/D+F or B+C/D+E configuration. My understanding is that the general idea is to have people at the front, middle and back i.e. you should stay in your row, but the exact seat in each row doesn't matter that much.
If I had been in the middle with someone in the window and the aisle empty, I would have moved to the aisle after takeoff, and stayed there unless the cabin crew specifically instructed me to sit in the middle again.
However not only did the person next to me not move, he also put his arms on the armrest and his right leg was beyond the edge of his seat encroaching into my space.
What do you think?