Shooting of any sort is an acquired and fleeting skill. Having that said, iron sight shooting requires the eye to focus on 3 locations at once, where using a scope has only one focal point. As we get older, this becomes more difficult as our eyes adjust slower for multiple focal distances. However, its my opinion that the people who gain skills of open sight shooting are generally much better shots than those who have not mastered this. Stuff like shooting gophers with .22lr or chasing a pop can around in a gravel pit with an SKS are invaluable shooting exercises, and i recommend them for everyone. My eyes are getting older and long shots with irons are getting more difficult, but I've done this for so long that rifle like my .22 and SKS just seem to aim by themselves to hit the target. I would also add that people who shoot only one box of ammo per year are not near as good shots as those who shoot five and ten thousand rounds per year. I demonstrate this for my hunting buddies several times per season.
Shooting is a fleeting skill as any sport, needs practice and ongoing challenges to be at the top of your game. Iron sight shooting enhances skills of quick target acquisition and depth/distance perception. These same skills also make one a better scope shooter. Some people really can't shoot worth sh*t because they've never really learned how to shoot. Iron sight shooting skills is a must have, and in some ways its a shame that scopes appear to be the easy way out, but at the end of the day they are not. Time in makes you a good shooter, not a scope.