Seat backs are a must. Helps define your seat space.
Totally agree. Either that or be very aggressive with maintaining your seat.
We had one or two games where 2 people were missing from our group and people on one side totally took over those two spots and started encroaching ours. We didn't want to buy seat backs since we were at the top of the lower section & had the concrete wall to lean against. So we learned how to ensure we still had use of all the seats we paid for, regardless if someone would be sitting in it.
One year (different section) we bought seat backs after the first game. We were 2 seats from the end of the row and the people there kept pushing into ours. They were
not happy when they arrived at the second game and saw we had seat backs.