If you select some text in the email you’re capturing, it will be added to the note field of the new action when you invoke the “OmniFocus 3: Send to Inbox” service. I find this very helpful as it allows me to capture key text from a potentially long email, keeping the note field clean and relevant.
On a side note, this same behaviour applies to most other apps. For example, if you select some text in Safari and invoke “OmniFocus 3: Send to Inbox” the selected text will be added to the note field along with the link.
Bonus Tip: In the Quick Entry window you can select some/all of the text in the note field, right or control-click, and select Simplify Style of Clear Style to make the text more readable.