In November, with crowds probably similar to late April, we had FP for the first singalong of the day and after they finished letting FP people in, there were seats on the far sides and in the very back in the "nosebleed" section, but I was glad I got a FP. Plus the seats at the very back didn't have backs to them (more like bleachers).
So do you have to? No. Will you get better seats and enjoy the show more with FP? Probably.
Also, they strictly enforced the 15 minute window for FP people. Once that time was up the shut the door and didn't let any more FP people in from that direction.