tonfa suggested I add a comment here.

Often, I don't want to resolve merges immediately after a pull. Rather, I want
the files left with merge markers, so I can come back to them individually later.

Unfortunately, if I forget some of these, hg lets me commit those files with the
markers still in place, unlike (some?) other SCMs. It should be checking for
this and not letting me commit until I've resolved the merges.