It occurs to me that, in several years of fighting for a more inclusionist stance on fictional topics, I've never been thoroughly explicit about why I care.
I am a PhD student in English focusing on popular culture. I have written and taught on comics, video games, television, films, and all manner of obscurities. I am, in this area, not just an editor of Wikipedia. I am a serious user of it. I routinely use Wikipedia to look up and check basic facts about fictional subjects - what episodes a minor character appeared in, who the creator of an obscure 70s DC comics villain was, what the plot of a random episode of something is. I use Wikipedia for background information to decide if something is worth looking into in more detail. I don't cite it, or use it in place of the primary source for close readings. But if I need a quick throw- away fact or to know if looking into something is going to be worth my time, Wikipedia is useful.
When an area that has been thoroughly worked on has not been decimated by deletion and notability police, it is an invaluable resource that often has not been duplicated elsewhere. There is no other place on the Internet where I can go for some of this information. When articles are deleted, the information and work that went into them are gone, and I cannot get at them. For a handful of shows with active Wikia projects the information gets mirrored elsewhere, but for a large amount of stuff, deleted is gone.
Plot summaries, cast lists, biographies of fictional people, etc - these things are not "fancruft." They are information that I use for serious scholarly research. Regularly and routinely. When they are deleted, my work becomes harder. When they are present, I can save hours of searching for minor details.
I am all for adding more in-universe material, and cutting summaries and the like to manageable lengths. I do not advocate scene by scene or chapter by chapter analyses. But on the other hand, it's tremendously useful to be able to get a general plot of a television series from the first episode up to the present. Useful. To real research.
When the information is easily verifiable (as in-universe information almost always is), the purpose of a notability guideline is to restrict Wikipedia to useful information. The "multiple independent sources" rule has, in the past, been a somewhat effective way of handling this. But we must not forget that the point of a notability guideline is to make sure that Wikipedia is limited to only useful and important subjects.
Plot summaries, episode descriptions, minor characters, etc are useful. They are important. To real, peer-reviewed research. We are not talking here about webcomics that end after three weeks, or about obscure garage bands. In most of these cases we are talking about television shows with audiences in the millions. People study these things. They are legitimate objects of academic research. And Wikipedia's coverage of them helps. It is an area where Wikipedia directly serves the public good.
Please stop deleting this stuff. Thanks.
-Phil Sandifer