1. yes, we can hide them during gameplay. But not during replays or while spectating. Skinners don't take screenshots while playing. They use replays in order to time their previews correctly.
2. click to dismiss wouldn't let you take two consecutive screenshots without those notifications popping up. not to mention it being buggy. take 3 consecutive screenshots then quickly dismiss them all at once. a few seconds later, you'd see them popping back up again.
3. If you think that it's not necessary to have the ability to hide screenshot notifications completely, don't you think it's also unnecessary to have the ability to hide friend status notifications completely?