I agree. The last time that DW & I rode RnR together, we noticed one guy in a distinctive outfit enter the SR line. Even though we were in the normal standby line, we boarded before he did. We saw him again, and there were probably 20 more single riders in front of him before the boarding CM. The CMs do a pretty good job filling the limos and only every once in a while do they need a SR.The only WDW ride I would not recommend single rider for is Rock N Roller Coaster. I have waited longer than the posted standby wait multiple times in the single rider line so I've learned my lesson there. A good rule of thumb is that if you can physically see the single rider line end on the ramp towards the inside queue area, don't get in line. Just hop into standby or get a Lightning Lane.
Also Space Mountain doesn't technically have a single rider line but when you are in the room with the standby/Genie+ merge point, if you tell the CM you are a single rider they will send you to a different line.
Yes, it was built but never used. The seat dividers were also removed from the ride vehicles so I suspect that queue will not be used.I think that Remy's was built with a SR line, but they've never used it. One time I was going through the LL queue for Remy's, and I noticed a sign for it.