It has to do with the wheel size, not the folding capability. 20" wheels transfer more vibration directly to the rider than bigger wheels. My girlfriend had a Giant Halfway 7S (no suspension at all) and after just a few miles on a downtube decided to trade and one of the reasons was the level of comfort.

Unless you are looking for a folder that is extremely efficient (since suspension will make you lose some power while pedaling) I would recommend at least some suspension if not a FS.

Just my US$0.02

Rafael