I think it sounds like a good idea, but until I have tried it I would not be willing to comment on its effectiveness. One of the ways I see it being a good idea is in competitions that require a fly line to be used. It may also help people who struggle to cast the long leaders used in French or Leader to Hand styles of nymphing. I will let you know when I have tried one
Also concerning the length, I think for a river and this style of fishing 45 feet of fly line will be more than adequate.
Just my opinion.
Frank