I recently read a short article about the use of a fixed length line system as a fly fishing teaching tool. I agree with much of what the author said about how it simplifies things for the beginner. The article stressed that it eliminated the need to shoot line and mend line.
I don't agree that it eliminates the need to mend/manage line on the water. I will say that having a long rod and very light line makes it alot easier. This allows the student to learn to recognize what happens when line meets water, to see the effect, and to be able to figure out what needs to be done in order to make the fly behave as the angler intends.
A fixed lenght line system certainly eliminates shooting line, but it does even more in terms of simplify the process. What I've found really helps a beginner is eliminating the need to coordinate two hands in order to successfully cast (even when you aren't shooting line). When folks are first getting started, it is indeed a pat head/rub stomach type of exercise trying to coordinate the movement of both hands. Eliminating the line hand allows a person to focus on using just the rod hand to make a cast.