If you have pages A and B, with B newer than A, you cannot merge it into A. (or if you do newer revisions are skipped).
Also you cannot just move/delete pages either since the move log revisions will control what the final page contents are.
We need a simple way of merging two pages into one.
A use case has been brought on the mediawiki-l mailing list:
to be able to consolidate 'forked' pages.
The workflow is offered like this:
If you know what you are doing you can do this via page moves. This is a major footgun so should be left to that emergent behavior not explicitly supported.