I get the impression that presently when you buy or sell SSFs the market maker just trades an offsetting position in the actual security. That is, if you sell a contract of 100 shares there will be a corresponding purchase of the actual 100 shares. So you're just paying someone else for their capital, transaction fees, and profit. Hence, spreads aren't good and won't improve until there is an active market between participants rather than market makers taking the offsetting transaction all the time.