True, but all the venues they've used so far could handle at least 40,000 seats without temporary structures. With many there was no need at all. I was basically looking at options beyond the south end for BMO. Sure it wouldn't be a lot but if you angle down from the permanent seating you could get approx 600 more seats on each the east and west side. No, some wouldn't have the best viewing angles but even in proper hockey venues you can't fully see the other end from certain seats right behind the boards.
Of course, totally forget that for any other sport. It would be a waste of time.