This argument does keep coming back! It's only really a problem when (as at Arsenal and Wembley) you get no choice in seat allocation and the sections are full. Then I would expect everyone to respect their fellow fans, a good many of whom will struggle to stand for 2 hours or be too small to see if everyone's standing. The "spoiling" of a game for someone who prefers to stand having to sit is nowhere near as great as it is for someone who doesn't have the choice and can't see the game they've paid to watch.
Beyond that, I've no problem with people standing if they're not blocking anyone else. Over the years, I've stood far more often than I've sat but, if I'm in a seat, I'd rather just stand when there's some excitement, not for the whole match! [Especially when the seats are as comfy as Arsenal's!]