Lol, I enjoyed reading this thread.
I'm a stickler for manners & politeness & going by the rules, so if I had to give a definitive answer I would say "no, seat poaching should not be allowed".
BUT, that said, I suppose I personally would not be up in arms if it was done in a polite manner. I myself was at a Mariners Baseball game once with my family, and IIRC me & my sister, et al tried to get a better view by moving up a bit, as we noticed some seats that weren't occupied during the beginning of the game, however the occupants were late to the game, so of course we politely apologized and went back to our seats a few rows back.
Seriously, if an event has plenty of seats open, and sometime during the event nobody shows, I would think it would be alright to
try, lol, but only if you purchased seats in that section. Otherwise, I feel it is very impolite & no-no to try to move to a whole other level, like up to the ice. And if the event is packed, no way. JMHO.
Lol, I recall this one time I was at a COI show with a good elderly friend of mine, whom has long passed (her mother used to take her to ice skating shows way back in the 30's!). Anyhow, she knew Oksana Baiul was my favorite, so I had planned to run down to the ice when she skated to take a few pics & also to hand her a present. Me & my good friend were seated about 15-20 rows up from the front of the ice, so I had a long dash to make downwards, lol, but I think I was still in my 20's back then so no problem. So I was sweating profusely anticipating my *big* moment, and then off I dashed so fast I honestly almost fell over the rail onto the ice!

What a wreck I was so concerned about everyone else, felt like a fugitive on the run, nervously sat on the stair so as not to block anyone's view as I did what I had promised myself!
Nowadays I don't think I would ever do something so rash again, but I've always been known to be a bit wild now & then, so who knows!
