Open irons will take an infinite amount of abuse when compared to a scope. If not, my Uncle would have issued me one with my M-16.
PS Junior:
You have not the slightest clue what shooting means when the target is shooting back. 
Though I've never had to bust a cap into someone, I do know that after around 600m it's darn near impossible to make out a standing sillouhette, and even harder when iron sights get in the way. Unless you're Superman, at those ranges a scope IS necessary. In addition, without any illumination near the target (night or a dark room) especially if there is light near YOU, a night vision scope or at least illuminated sights are necessary. When reaction times are a factor (such as in urban warfare) aligning one's iron sights will take more time and as a result of closing one eye, give less peripheral vision. Though iron sights are tougher than scopes, an AK-pattern rifle is tougher than an AR-15 pattern rifle, but the US Army still uses it. If the Army, which you cited as a source for credible armament information (your uncle was issued an M-16) then in both the cases of the AR-15 pattern rifle and the scope one can argue that toughness can be traded for qualities such as accuracy and rate of fire. Though scopes have their imperfections, when used properly they are an excellent tool, just like iron sights.
