In terms of gameplay mechanics, old school zoom aiming achieves the same thing as ADS. Is getting to press your nose against virtual firearms really that important in a game about shooting werevolves and vampires?
I blame this on the proliferation of Call of Duty over traditional scifi and fantasy shooter franchises. You can fucking ADS on an assault rifle in Halo now 
I'm fine with not using the iron sights if there's no further level of aim (counter strike) or if there's not really any iron sights on the guns (Halo). But if you're in a first person game, it feels weird to just zoom in when there are iron sights on the gun that the character would be using. For me, it's about being more immersed in the game play. It's like when you look down and can't see yourself. Yeah it doesn't break the game but it definitely takes you out of it a little bit.