
I find Joiner fascinating because it's also (an unintentional?) commentary on TF2 design styles: rooms are still composed mostly of simple rectangular planes that join at 90 degree angles -- that's the actual functional level geometry, but a typical player would recognize that as undetailed and thus as an unfinished / crappy map. What Johnson has made is not a "make level" button, but rather a "make detail" or "stop players from whining" button. The purpose of these struts is to cover the surface in a sort of greeble, so the player won't be distracted in comparing its perceived quality against other maps with "better" detail.
Surface detail is a paradox. It is "necessary" to exist in front of the player, but it exists to be more or less ignored.