It's a question that I've always wondered, and have never asked...simply because I am very much a big proponent of single rider lines! But I recently spoke to a family who disliked being placed with single riders because their young-ish kids enjoyed rotating who gets to "ride by themselves" (they are an odd-numbered group), but many attractions don't let them.
Admittedly, being placed with a single rider is always a bit awk. Heck, even being a single rider is a bit awkward, but at least if you're the single rider you expect that/prepare for that.
I'm curious if anyone has ever tried this! Please don't attack me - just an honest question I'm curious about. I'm sure lots of people who don't know any better must complain about it all the time, right?