The current revision id of a page is called 'revid' in the Action API. When requesting 'ids' from rvprop in the Revisions API, the previous revision of a page is called 'parentid'. Thinking about XML and node hierarchies, I find that terminology to be a bit of a misnomer. Rather, the term from the RecentChanges API, 'old_revid', both more clearly relates to 'revid' and implies chronological ordering rather than hierarchical.
Use case(s):
Both of these queries refer to same recent change on the same page. If a user has previously retrieved a copy of the page via the Revisions API and wants to check the RecentChanges API for updates, the fields 'parentid' and 'old_revid' are not immediately comparable.
Benefits (why should this be implemented?): Changing 'parentid' in the Revisions API to 'old_revid' will make it easier to combine different kinds of API queries for both getting page content and determining when/if that page content has been updated, as well as making it more clear initially that it refers to the previous revision of the page.