Please do not show the private note option to users who use email issue and do not have the ability to user private notes.
When a user clicks email issue they get a prompt to add private notes to the body of the email or not. Users who do not have the ability to user private notes should not be presented with the option to include private notes. No mater what if the user does not have the ability to view the notes, then the private notes are not added to the email body.
To add to this: Even if the user can use private notes, the message box is a little confusing as OK, the default button, adds private notes to email. This is very bad as agents can just click through by accident and allow the end user to see all private notes.