If messages come from one known-good source (e.g. firmware updates), a digital signature is a valid message authenticator. Where this falls down is in multi-use

If messages come from one known-good source (e.g. firmware updates), a digital signature is a valid message authenticator. Where this falls down is in multi-user settings (e.g. iMessage), where secret-key authentication via MAC becomes a necessity. I'd still recommend always to use an authenticated cipher, since the above distinction can be very fine.