The problem I have with ADS is that as soon as it is added to a game the developers usually add some innate bonus to it (accuracy, extra zoom or damage) so suddenly it becomes a detriment to not use the sights. And don't throw realism at me because I'm pretty sure a shotgun's spread doesn't magically decrease when you hold it closer to your face. Some guns are meant to be aimed down, like snipers and assault rifles, but some guns aren't, such as pistols and shotguns at close range. Plus if ADS is implemented some guns become god awful because their iron-sights are terrible, at least make them customizable.