I can see it being useful in two circumstances:
1) As part of the oversight right, in order to edit an edit summary without
hiding the entire revision
2) A right of a user to edit their own edit summaries, if the edit summary
is blank
Since it's possible and at least some people are interested in it, I don't
see the downside of making it available in MediaWiki even if most Wikimedia
projects might not use it.