I use it.

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.

We are a small web development company. I'd say that about 20% of our clients used this feature because gmail WAS their email client. They did't want to use a device-based mail client.

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.