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When the Wikipedia community is attempting to come to a consensus whether content belongs on Wikipedia or not through the "articles for deletion" process, one of the reasons commonly heard to keep content is that the content's "interesting."
What if we wrote an encyclopedia and no one came? For content to be interesting is a good thing — it suggests that the information within was presented well-enough to capture a reader's interest. People have to want to read what we write here — obviously, all editors should try to make their writing interesting.
Interestingness, however, is not a useful way of measuring whether or not content belongs on Wikipedia.
Wikipedia requires certain things of its content, and these requirements are among its core principles, making them for the most part unable to be countermanded or superseded. Wikipedia requires that its content have certain qualities — for example, content should be verifiable by cited sources, and should not espouse a particular point of view. Wikipedia also requires that its content not have certain qualities. It should not be a place where brand-new ideas and theories are created, for example, as exciting as those theories can feel to the editor or to other readers. And content should not be FAQs, quotations, travel guides, memorials, breaking news, directories, or instructions.
The problem is that content can be considered interesting and yet be any one of, or all of, these things — making the quality of how interesting content is useless in measuring whether or not that content belongs on Wikipedia. Content can be interesting and yet unverified, uncited, and biased. Content can be fascinating and yet be a brand-new idea (in fact, brand-new ideas often are). Content can be captivating and yet be a collection of FAQs or quotations, or a manual, or a travel guide. Because of this, interestingness really has no effect on, and offers no answer to, the question of whether the content in question fulfills the basic requirements that Wikipedia demands of its content.
Interestingness is also useless as a metric because it is an intensely subjective metric, one that changes not only from peer group to peer group but from person to person. Because of that intense subjectivity, there is no practical way to build a consensus on the interestingness of content, and, as such, the quality has no place in a process that relies upon that consensus to decide whether or not content belongs on Wikipedia.
New articles are created on Wikipedia seemingly every second. Putting aside those articles created by bots, out of a sense of duty, or as vandalism, those articles' creators are interested in their creations — otherwise, they would not have created them. But we cannot take those articles being of interest to their creators and, using that, draw conclusions as to the level of interest any other editor will have in that article — an article on the nameless butler that appears on page 497 of famous author Jane Smith's epic Pencils and Pens may be of interest to Smith's fans, but, again, we cannot take the interest of Smith's fans in the article and, from that, form a prediction about how much the article might interest others. Although contributor John Smith may find the one-paragraph article he just wrote about his dog quite interesting, we cannot take John's interest in the article and, from that, form a prediction about how much the article on Fido might interest others. Instead, we must rely upon more objective metrics, such as notability, to decide whether or not his dog deserves an article — if John is a current or former world leader, his Scottish Terrier might indeed be notable enough to get her own Wikipedia article (but then again, his pet cow might not).
In conclusion, interestingness, although intrinsically valuable to and desirable for Wikipedia content, is a quality that is unpredictable and extremely subjective, and one that can further offer no metric as to how well content measures up to Wikipedia's content policies.
As such, the question of whether or not an article is interesting cannot have any weight, and should not be offered nor considered as supportive reasoning, in a discussion of whether or not that content belongs on Wikipedia.