Im with Thor on this (unless i can actually see how it works) - ok the seat and the rear may be linked together in some way, but there are situations as Thor says when you need a ridgid seat post.
Also a folding stem 'clamp' that is only fixed by a cable !!!! .... when the steering its self is free to rotate .P-lease !!! If anyone has ridden with a loose steering clamp they will know the fear. I have, forgetting to do up the steering clamp on the brommie - I would of expected with reasonable balance to have been able to steer ok but infact its totally counter-intuitive, I fell off in about a yard (luckly slowly) ..... NOT recommended it was an awful experience.
The same would apply to any folder with a folding stem, ie Dahons, Downtubes, Birdies etc.(most bikes, excluding other Bike Fridays, swifts and airnimals which slide off).