Four fragmentary copies of a four-chapter book don’t demonstrate much. Do entries like this exist only to make the list look complete?

The list is complete on purpose. The claim is about the whole Old Testament, so every book’s status is reported, including the thin ones and the two that are missing. Ruth’s fragments do agree with the later text where they overlap, which is a small data point in a large sample. Omitting the minor books would invite the charge of hiding gaps; reporting them, thin or not, lets you see the real shape of the evidence.