When you delete a user with Esri Access disabled, is the username available for anyone to claim? If someone happened to claim it after you let it go, you wouldn't be able to recreate the username.
Alternate scenario: if you delete a user with Esri Access enabled and they later come back to your organization, couldn't you invite them to use that same account they still control? (The "invite members to join using an account of their choice" method.)