This is just my opinion, but I think it's one of those cutesy gadgets designed to catch fishermen, rather than fish. First off, fly rods are designed to cast with the weight of the reel on them, and the handles are designed to be at the fulcrum, so the rig is balanced. The weight of the reel is the counter-balance to the weight of the rod in front of the handle. The reel weight actually helps you to cast more effortlessly, and improves the loading of the rod tip. I think removing the reel would make the rod tip heavy, and adversely effect it's performance. As far as tendinitis, I don't think this will help, because you still have to make the same movements to cast, and now, you will be doing it with an unbalanced rig, which may actually make the problem worse (personally, I would love the opportunity to get tendinitis from casting too much.......I think he is just bragging...I know I would be...) The only neat thing I see about this unit is using it as a rod-holder, but most fly vests have a velcro loop than can be used for the same thing, and much cheaper. My fishing pants also have a velcro rod holder on the hip. You could get some velcro at Walmart and make your own rod holder on any of your clothes, coats, pants, vests, etc...for just a few dollars.
This is just my opinion. If you like those kind of gadgets, then by all means feel free to indulge yourself (And I cannot talk...I have every kitchen gadget known to mankind....). But this is not something I would use. I like my fishing gear just the way it is. My only complaint is that I don't get to use it enough.