An example for is if you Answer, and then go back to edit your answer within the first grace period. In such cases, the edit summary is never displayed, so why provide a box for it?
The field for the edit summary is shown because it is not known when you are done with the edit; it could be you are done before the grace period for your very first edit is over, or it could you are done after the grace period.
What could be done is enabling the field for the edit summary after the grace period is over. Considering it happens for the very first edits done in the grace period when a post is created, I would say that changing the code is not worth the time spent.
The worst that can happen is that you thought have done the edit during the grace period, you don't write a summary, and you notice that the edit was completed after the grace period. You can still change the edit summary by clicking on the "edit" link. What you need to do is entering at least a new online, but if the feature request in Unable to retroactively add edit summary during edit window is implemented, that would not be anymore necessary.