IIRC, they also give you bonus points if you refer someone to HGVC who completes a purchase. However, you would not be doing them any favors for doing so.
You can use bonus points not only for open-season, but also other perks, RCI trades, Hilton hotel reservations, conversion to HHonors points and maintenance fees.
HOWEVER, they will soon be gone, and you will be left with a T/S for which you paid way too much.