As others have stated, single rider line is a nice customer service offering. When we were there, as a family of 3, a couple of weeks ago, we used the SRL. Comparatively if SRL were not available, each time we rode there would be one empty seat. Why not fill the seat? Could you imagine how people would complain if there were 4 empty seats on each train? As far as the cast members running the single rider line, I have not seen a special CM managing only the SRL. There are some really awesome CM who know what they are doing and can move those lines and fill the seats. In fact one was so good on our last trip, I stopped at Guest Relations to let them know. ( I try to do that at least 1x per trip--I figure they hear many more negatives than positives so it is a little way I can brighten someone's day).
Now, if they had a SRL at TSMM, I'd be thrilled. Again as a party of three we have an empty seat when we ride.