I'm not a fan of the summary option on body fields in Drupal. I've never really got on with how users interact with it or the content is produces.
The field type I'm talking about is called "Text (formatted, long, with summary)" and it appears as an add-on summary field to a normal content editor area. It comes as standard on Drupal installs and appears on all body fields. The field type has it's uses, but I often find that the content it produces is unpredictable and doesn't have a great editing experience. I have even written articles in the past about swapping the summary field to a fully fledged wysiwyg area for Drupal 7, which worked on the project I implemented it on.
Let's look at the body summary field in Drupal and see why I have a few problems with it.