Using XOAUTH is useful when you want to send e-mails on behalf of your users (eg. as if the sender would be the user, not your application) - instead of asking their passwords, you can ask them to authorize your application through the OAuth mechanism. The resulting client secrets are known only to you, valid only for accessing the e-mail of the user, nothing more and the user can revoke these client tokens any time they wish.
If you do not want to send mail on behalf your users but with your own credentials, then there is no point of using XOAUTH.