I really need one of these!!
There's a way you can get the middle chain off very quickly if the need arises, only on straight downtube bikes (the engine fairly high) and using the optional tensioners but it's something that I need for convenience and legal purposes. With the middle chain off the bike pedals with no additional drag at all and cannot be started.
The jackshaft allows the use of the freehub on the right hand side.
This has two big advantages, firstly it allows the bike to freewheel without you pulling in the clutch (but you can still start the engine with the pedals, no need to get off and pull start) and puts less drag on the bike when it's freewheeling because it isn't dragging the chain around (or momentarily engine braking while you react and do the declutching).
The second advantage is it allows you to use derailleur or internal hub gears.
There is a disadvantage of power lost into moving those three chains, six sprockets and up to five or six tensioner wheels. Top speed would be lower than
if you were geared specifically for top speed on a single speed bike with one chain, two sprockets and one tensioner.