The Slingshot used a sprung cable instead of a down tube, and had some sort of fiberglass hinge at the rear of the top tube. It seems like it should have been far too flexible to be useful, but I've heard of some folks who really liked it.
I'm not sure how there is enough torsional stiffness in that frame to work, and the marketplace seems to have decided that it wasn't a good idea, but some folks liked it. I'd love to hear from any BF folks who have experience with it.
... of course, I have a friend who loves his Schwinn Varsity too, so I guess you can never tell what someone might like.

Steve in Peoria