Pretty much repeating what others have already said, for the most part:
The main line is The line.
The singles line (if there is one) exists solely for the purpose of filling seats that would otherwise go up empty. Thus: when a single joins a group that is in the main line (and that otherwise would've filled less than a whole chair), it has no effect whatsoever on how quickly anyone in the main line gets to load.
It follows that the proper etiquette is to give people in the main line priority: the exact opposite from what the guy said. Somewhat on the other hand, if you're a single and got in the main line, and got all the way to the front without joining a group, either:
- the main line was sufficiently short that you chose it, and didn't have time to join a group before getting to the front, or
- you were kind of remiss to stand in line for awhile without joining the group earlier.
If the former is the case: the etiquette point doesn't really matter. If the latter: you're cavalierness kind of makes it a case of six-of-one, half-a-dozen-of-the-other (or, I suppose, four-of-one, a-third-of-a-dozen-of-the-other) etiquette-wise.
Additional thing that follows: It's never appropriate for four people in the singles line to take a full chair.* Well, okay, unless they're the only people waiting.
Aside: in the old days, when chairs were almost all doubles (and lines were much longer!), there weren't singles lines. You pulled up to the back of the line and yelled "single!" If someone already in the line was single, you got to jump in line with him; if not, you got in the back of the main line and waited for someone else to come along and yell "single!" It was quite bad form not to yell, or not to answer a yell, so that ordinarily an arriving single would join someone fairly near the back of the line. If someone went all the way to the front and loaded a chair by himself, he was liable to be beaten with ski poles.
As to the contemporary semi-corollary (people intentionally failing to fill chairs when there's a line): the more egregious cases may very well be teenage snowboarders, but that doesn't really have anything to do with the number and dimensions of their sliding boards. It just has to do with the fact that teenagers have a talent for doing egregious things, and snowboarders are over-represented among teenagers.
*Or three, if the chair is a triple ... or six if the chair is a six-pack, just to cover the universe of possibilities.