It works for a baitcaster or spinning reel because in these cases the fisherman exclusively uses the reel to retrieve line after a cast. On a fly rod, more often than not, the only time you use the reel is when a fish is caught or when you are ready to stop casting for one reason or another. The rest of the time, the line is stripped in but left off the reel while actively fishing. The only way to maintain control of the stripped line is under the finger of the casting hand. How would you do that if the line was not forced past that casting hand. Assuming you could do it, the reel would constantly be in the way of grabbing the next length of line to strip and, I would imagine, cause constant tangling.