Not sure I get you. 

You mean prepending the user email to the message body in all cases? That wouldn't hurt but if your SMTP allows you specifying any "From:" address you want there will be no benefits either.
I opted for not adding anything to the message body unless there is some reason to add it.

Maybe I wasn't clear myself in the initial description too.
Gmail by default will replace the "From:" address with the address of the authenticated SMTP user. It will also strip the "Reply-To:" address.
If you use a "From:" address you have previously verified that you own, it will not change the "From:" address but it will still strip the "Reply-To:" address.

So, if you use ``DEFAULT_FROM_EMAIL`` the user email is added to the body to make sure the address won't be lost.

On an afterthought, the problem may be solved more elegantly by simply passing the user email and ``DEFAULT_FROM_EMAIL`` as context to ``render_to_string()`` and then use a custom email template.

I could implement this approach if you find it better.