Editing summaries aren't peer reviewed, moderated or otherwise subject to the protections that posts and comments are. Therefore they can sometimes become snarky and harmful. Should an otherwise-satisfactory edit be rejected due to a harmful edit message? And indeed, once it is accepted, there's no way to edit or delete it.
Well, ♦ moderators can edit edit summaries. If you see a particular harmful one, you can flag the post and ask them to change it.
Should an otherwise-satisfactory edit be rejected due to a harmful edit message?
Yes - even if the edit itself is good, you'd think you'd be able to solve that by choosing Improve edit, change the edit summary (and something else to make sure it's an edit), but that would only apply to your edit and not the suggested one (which generates a separate revision). And approving the edit gives the suggester +2 reputation, and we don't want to encourage this behaviour.
Anyway, it's good that you pay attention to them while reviewing, because it's very easy to overlook them.