Silk goes on relatively easily and provides immense added strength. Silk has more than 10 times the tensile strength of steel by weight.
"Glueing" the silk to the foam with WBP paint essentially fiberglasses the plane with a cloth that is far stronger than fiberglass. You get a very stiff and strong model for a relatively modest increase in weight. The silk and WBP method is simply lighter than fiberglass and epoxy.