Those chain stays were, I believe, left over fork blades. The intent was to eliminate (or at least reduce) chain slap (and possibly chain suck, wherein the chain got sucked between the tire and the stay - Shimano addressed this with the infamous "Shark Fin") when traversing rough terrain. Giant did something similar only from the other end of the stay, by lowering where the stay attached to the dropout. I think the Schwinn design was more elegant than both the Giant and Shimano's Shark Fin, for that matter. Now, had Schwinn used curved, or "banana", seatstays I imagine the ride would have been very compliant indeed.