I've come across several edits by >2000 users which contain no edit summary (just the automatically inserted "deleted xx characters in body").
Should I assume that edit summaries are not regarded useful when the edits don't need to be approved?
This question and its answers/comments seem to imply otherwise.
They are not just for the benefit of reviewers. They're not even just for reviewers and others who might question if an edit should be kept or partially reverted / completely rolled back. They are for anyone reading the revision history.
Edit summaries appear in every post's edit history, "for all time." They are viewable by anyone who can view the post. Edit histories are not indexed by search engines, but any person can look at them.
For example, I improved the appearance of an ordered list in this answer with the edit summary improved formatting. Now suppose someone clicks the link to view the post's revision history:
Then they get this page, which shows:
This does not necessarily mean that submitting a blank edit summary is universally bad. Sometimes an edit is simple, the justification is easy and non-controversial, and it speaks for itself more clearly than a summary. In these situations, it is acceptable to submit a blank edit summary, because the system automatically adds syntax-based edit summaries when none is provided. (Like edited title, edited tags, and removed 13 characters from body.)
However, as Oli says, it is never wrong to provide an edit summary. (Unless the edit summary is wrong. Or provides no information. Like the improved post edit summaries I complained about there.)
I would say edit summaries are there to justify an edit that may not be completely obvious. Most edits we do are formatting or language fixes so they are obvious. They don't need a summary written out for them.
But occasionally I'll do an edit that changes the meaning of something, either because a fact has changed in time or it's just not quite correct. In these cases it's important to provide a summary, not only because it might get reviewed but because the author (even if that's you) might want to review the edit at some point in the future. A summary provides a log.
If you're unsure, consider this the other way: edit summaries are never considered un-useful; providing a summary can never hurt.
