Kibana won't let you create a user without specifying an email address (to which it applies some, not checked what exactly, syntax validation).
Why? - isn't it up to me whether I want to specify an email address or not? What's it going to do with the email address? Why should I have to specify email addresses for users that aren't even people? - typing in "firstname.lastname@example.org" seems a bit unnecessarily tedious.