Well, for one thing they wouldn't know anything about Koakuma's unless they already joined. We don't exactly advertise that there even is a completed works of fiction section on the site. Heck, if I were a guest browsing the library catalog, the first thing I would notice is the fact that half the stories are missing, which looks sloppy on the outside. Part of the problem is that the site doesn't distinguish between broken links to missing pages and links to parts of the site that are member only (they appear to share the same screen), so to an onlooker their first thought would be about how dozens of stories are missing, not that they should join to read dozens more stories. That part of the site needs to be advertised in some way, even if guests don't have access to it, simply so that guests will know it exists.
I don't consider Tengu's point completely fair because guests wouldn't know the completed stories section exists, and I think they would be more likely to join because of the completed stories are good than the incomplete ones. And I don't see why the completed stories should be restricted to people who already have a committed interest. Truthfully, the reason I got into the site in the first place as well as started taking an interest in writing at all was because of the fanfics on this site. I probably wouldn't have registered or even have had an interest in the site in the first place if not for stories like Frame of the Undying Fortitude and Journal of Patchouli's Loyal Assistant (my membership predates Koakuma's). I consider Koakumas to be a very good demonstration of what the library is about, and a selling point for the library as a whole. Hiding it from the public does us a disservice, if you ask me.