I think that if you shoot instinctive, then one arrow ingrains the idea of picking a spot and putting your whole concentration on it for one shot, as in hunting. It is very hard for an instinctive shooter to sit there and concentrate at that level for a group of arrows. Also, you don't want to get in the habit of, in effect, consciously "gapping" to correct for high and low misses, which you will, I guarantee, do if you shoot several arrows at a time. On the other hand, I think gap shooters are well served by shooting groups to ingrain the precise gap needed at a given distance, maybe even doing the Byron Ferguson thing and having two aim points, one for the arrow and one for the focus (actual target). Could be wrong, but makes sense to me.