I always thought the intended purpose was having all the seats on the train full for maximum throughput of folks who want to ride. I'm sure there are some who beg to ride together even in the single rider line! If the cast members in charge are good, there should never be more than 1 or 2 people from the single rider line given a seat on any train. And some trains should have no singles. I can't honestly say I've looked that closely at the way they handle it. I just know earlier this month, when we rode EE, the single rider line was really long.... have to think the regular standby line might've been faster!