Consider the following scenario:
1 - I make an edit using VisualEditor
2 - I click Show Preview.
In this instance, the edit summary is not shown in the preview. The main problem is that I might forget to initially add one; I may click Show Preview first to ensure that it looks fine and then immediately press Save. Additionally it does not give an opportunity to review any markup that I may have added to the edit summary (used to be possible in the wikimarkup era).
It would also be appreciated if a feature warning the user if the summary is blank and Save is pressed (after previewing), assuming that the edit is not minor).