I prefer using Fastpass over single rider lines, since it's just as quick and usually seems quicker.
More importantly, I prefer Fastpass over the single-rider queue because when using Fastpass (or standby) your "party of one" is still considered a soverign party treated with the same coutesy as a party of 2 or more. In other words you can tell the grouper "I'd like the front row, please" and they put you in the front row line. The single rider queue is there simply to fill seats and you have to sit wherever the empty seat is. Since I'm a stickler for requesting the front (and sometimes also the back on coasters), I personally don't use the single line, but if someone doesn't care where they sit and doesn't have a Fastpass, I suppose the single rider line could save some time provided it's not backed up up too much.
Regarding the original thread question, I live in the area seasonally by myself so most of my park visits are alone except when family and friends visit. I enjoy the other advantages others have mentioned, such as the spontanaity of being able to do exactly what you want whenever you want, and also the ability to make good time and to get to do more attractions. I do enjoy sharing the experience with others when it happens, but also enjoy the solo experience, so I wouldn't want all of my visits to be either one way or the other.