They might be a little sturdier, but I`m pretty sure all the bikes discussed here are fairly sturdy anyway. One reason I`d prefer a demountable frame to a folder is that it gives you more liberty to arrange the pieces in the most compact way possible rather than being tied together by a hinge. Take down bikes also look more "normal", and the couplings are probably slightly lighter than a hinge system. I think the main disadvantage to couplings is that they require more time to pack and unpack than a hinged bike.