Wow, interesting! I have not seen shaft-drive bicycles before. Motorcycles, yes, but bikes, no. On a motorcycle, the shaft-drive can be a good thing, esp on a touring type one, because of being sealed and easy maintenance. However, you will notice that the racing type motorcycles are always chain-drive. I have been told that this is because a chain-drive more rapidly transfers the energy to the wheel than a shaft-drive does. If the same holds for bicycles (don't know why it wouldn't) I would guess that shaft-drive would be somewhat less efficient for that reason. Just guessing, here, though I do miss my motorbiking days