On some wikis, subpages are a legitimate way to organize pages in hierarchical trees which is complementary to categories: actually, where they are not, subpages are not supposed to be enabled.
Listing all subpages makes the list of uncategorized pages completely useless for categorization work.

For instance a couple years ago I categorized all pages which needed to be categorized on Meta-Wiki and even after this work there were more than 1200 pages listed there, of which only 65 actually needed to be categorized ([[m:User:Nemo bis/Uncategorized non-subpages]]) and a half shared a handful common basepages (cf. [[m:User:Nemo bis/Uncategorized subpages of non-common basepages]]).

Version: 1.20.x
Severity: enhancement