But filling gaps is a courtesy to people in line. The rides will work just fine with gaps. If Disney doesn't want to give away the spots, they won't.Because Single Rider has always been used as a means to fill in the 'gaps' created by uneven groups, never as a primary line.
Besides, single spots might be in high demand once they start selling spots. If one person in a group is the one really pushing to ride, maybe they only buy a spot for one, and the other group waits for them. The wait is short, and the cost for one much more manageable for many.