90% of the time when I find an empty middle, I'm the first to use the tray table when drinks come round. Maybe I just happen to always end up with infrequent flyers who don't know that it's common ground, but quite often they won't use their half of the open table, preferring just to use theirs. If it's just drinks/sandwich, I won't open my table at all, just use my half of the middle.

I rarely use the seat (apart from maybe a jumper). My small carry on will go into my side of the middle seat footwell.