It troubles me when people say a line is "playable at club level"

There are two approaches to chess openings: fight for a theoretical advantage or just reach a playable position.  Obviously, fighting for a theoretical advantage by definition means knowing and playing theory.  Playable positions, by contrast, can mean anything from the Four Knights (a mainline but not especially popular) to the Glek System (neither popular nor a mainline) to the Latvian Gambit (likely unsound).

The last one is most interesting, because even though the Latvian Gambit is dubious, it creates positions that are unbalanced and unique, which is a recipe for blunders ... for both sides.  The position devolves into chaos, and at this point, the better player will win.  That's a good thing.  You don't want to win because you memorized a sideline better than your opponent; you want to win because you are better!

As you get to master level, these openings have less appeal.  If you are a professional, others can prepare lines against you and really force you to suffer if you play even a slightly dubious opening.  That's part of the business of competitive chess.  Below that, though, you can thrive on creating unique positions and simply outplaying your opponent.  Isn't that what we all want?  To win games without worrying about x-number of theory moves?