I think VERY few people come to Disney alone (or at least get in a queue alone) but a fair number of people are willing to split their group up if it means they can ride significantly faster. For those willing to split their group just to ride significantly faster, though, I doubt many groups would be willing to save 5-10 minutes to ride slightly faster but separated when they have already waited 60/90/120 minutes in line already. If the Single Rider line just utilized the FastPass line and you went right to the ramps (there's PLENTY of ramp space to hold a good 30-60 mins of Single Riders), I bet you'd get a good number of people using that line to avoid waiting in the main line.
Problem with the set-up now is that, as a SingleRider, you wait the full queue and get put into the SingleRider line at the end and when they are looking for one or two riders to complete the fill of a theater, frequently that means you get put in seat 15 or 16. While the edge seats in FOP aren't anywhere near as bad as the edge seats on Soarin', they're still not the best seats and if I'm going to wait the full queue, I want just as much a chance as anyone else to get put in seats 7 through 10 (if I was able to wait an abbreviated time in a dedicated Single Rider line, I'd be much more willing to accept that I'd always get an edge seat).