It's weird that they are actively discouraging us from using external mail inside gmail. Like, you would think they want us sucking down all our scannable data into their system and staying inside the wall.
I am happy we will no longer have to support this. Losing the POP access a couple years ago got about half of them to move, and this will kill it off.
It is useful for anyone who has a personal domain from which they'd like to send/receive without needing to migrate off Google entirely. My registrar gives a few email addresses with SMTP support. Then I use GMail's send-as to remain in gmail but still use my name@mydomain.com mailbox.