While recently reading a book unrelated to archery I stumbled across a section that provided an explanation, at least in my mind, as to why one can shoot totally instinctively and be just as effective as another using a well defined aiming system.
It has to do with which side of the brain you are using in the process. The left side of the brain is our quantitative, verbal, calculating, discrete side of the brain, while the right is the spatial, relationship-based, feeling, non-verbal side of the brain.
Our left brain tends to dominate the right and therfore we will often as humans tend toward a "formula" rather than a "feel" to complete a particular task. Studies have shown however that when the right side of the brain is allowed to take control and use the more non-quantitative spatially-based skills it can be just as effective as the more calculating left brain.
So what's my point? Those of you who have mastered some sort of aiming system are using primarily the left side of your brain. Those who claim to be purely instinctive, have learned to overpower the left brain and are using predominantly the right side of the brain. Further, I have found that when I try to mix the two styles my shooting goes to heck in a handbasket really quick.
Now mind you this is coming from someone who has occasionally been told he only has half a brain, so to be safe I would just ignore it and go back to work.