I think that the main argument that isn't simply a matter of storage costs (which probably does kick in at some point) is simply a matter of brand-management. If the average Wikipedia article is a discussion an incomplete and poorly sourced article about Pokemon then Wikipedia gets a reputation as poor source of information. It actually had the reputation pretty strongly early on, but it's reputation for reliabity has actually improved over the years.

I suppose that there is also the inverse problem where poor articles can use Wikipedia's relatively good reputation to give visibility to some pseudoscience or conspiracy theory. If it's really obscure, the only people who will be writing articles about are will be proponents who will then link to the article from their closed Facebook groups. It won't necessarily be obvious to non-experts what's going on so deleting certain kinds of articles could be a protection against that as well.

(I'm not any kind of Wikipedian btw; just speculating off of the top of my head.)