You'd be surprised. You shoot nothing but open, iron sights for years and you shoot better with them than with a scope. Me? I get confused when I look through a scope. If I can see my target with the naked eye, I'm better off with a buckhorn and a blade.
I agree to a point.
That's all I shot with for most of my life; however, if I were going after people and planning on taking shots of two hundred yards or so, I'd always opt for a good scope.
Case in point:
You've not seen a sniper in the military using open sights since they quit using the Springfield '06.