Provide the ability to mark an edit as minor, meaning the update time is not modified.
Because your readers do.
If you are making minor changes (spelling corrections, category updates) to old posts without this plugin, your posts will be marked with a new updated date in your atom feed. Readers will then see your posts reappearing in their feed readers. Additionally, if your posts are syndicated (e.g. using Planet or Venus) they will reappear at the top of the page.
It is difficult to conceive of an algorithm that can correctly decide what constitutes a minor edit. Ultimately it is your decision as the author, based on whether or not the change you're making requires the reader's attention again.