I guess it depends on the camera. I'm happy with my relatively short, double extension Korona 7x17", because it's light and portable, and I see it mainly as a wide-to-normal panoramic camera for landscapes, some still lifes with a normal lens maybe, and full length portraits, all of which I can do with the extension I have. 11x14" I think of more as a portrait format, so when I had the bellows replaced on my camera, I wanted the maximum length for the rail I have (around 34", if I remember correctly--about 10" longer than the original bellows on the camera).
Check the classifieds here and at the LFphoto.info forum for Tri Tran's 20x24" Chamonix, which has a detachable bellows extension. I think for a convertable camera, that could be an ideal solution.
If you wanted to build a monorail camera around the Sinar P/P2 system, instead of reduction backs you could make format conversion kits, each consisting of a back that clamps to the post on the rear standard bearer and a tapered bellows that could be fixed to the back and would attach to the front standard, which is the same for all formats, so you would have an appropriately shaped bellows of suitable length for each format. And if you occasionally need more bellows, you just add another front standard (usually you would use an F/1/2 front standard as the intermediate standard), more rail, and a standard 4x5" Sinar bellows between the intermediate and front standards.