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Tape is sticky, normally single-use, disposable and not swappeable between bikes. Once you use a spare tube what do you do with the flat one? Ditch it? No thanks. Tying it down is better but still a bit of a faff and doesn't cover as much of the tube, this option is more protected.
Cost? Meh :) Anything related to bikes is £free :)
The double cage is just another option. I know some people who have already used their own version of that kind of double cage so there's definitely a small market for it.
hippy-
I wouldn't use tape either, I grew up using old tubes as bungees, so I'd have a piece of an old one cut and use it to tie a spare one to the bike. Very swappable between bike, non sticky, no need of bolts/holes, cheap and can make many from one single old tube. ;)
Edit: I obviously wouldn't ditch the old tube, that's just more bungees!
giofox
I think it's not optimized, if you just need to carry a spare tube down there you can just tape it, or even better wrap it around and tie it. I want the whole tool bag down there, with side access.
The long brackets are useful to move around the bottle cages, but not cheap.
The double bottle mount is bogus, unless you have chopstick thin legs, I don't see how that won't get in the way when people struggle with the width of frame bags alone. I'd use longer screws, mount the single cage on it, and use the side holes for fitting other things (some tools perhaps)