**UPDATE**
Our customers needing Domain Admin-level impersonation found a NEW REASON: Multi-Factor Authentication.
Currently, admins access user mailboxes by viewing the password, then logging in as the user. HOWEVER, when a user has 2FA/MFA enabled on their mailbox, the admin cannot login unless the user provides an Authenticator PIN code at that moment.
Admins handle email support TICKETS, not phone calls, and are not able to work these ticket requests without first getting the user on the phone and coordinating the admin login with the user reading back the PIN code.
This makes timely support very difficult-- and IMPOSSIBLE for the overnight support team to do anything needing mailbox access, until the next morning when they can make contact with the user first. Admins NEED the impersonation option to handle support tickets, without having to chase down each and every user who has a support request pending.