Canting the bow does change the point of impact of the arrow, but not as much for a traditional bow shot off the shelf as for an Olympic bow shot off a raised rest. Traditional archers learn to adjust for this, within an acceptable range of error within the distances we commonly shoot, i.e., keeping the arrow within a softball sized area at 30 yards or less. Clearly, if you were trying to do the same thing at Olympic distances, you wouldn't want the additional variable that canting the bow introduces. Perhaps less clearly, as traditional archers, we don't always have the luxury of standing tall and shooting with the bow in a vertical position.