It's not wild at all. It's just behaving like any other MUA but with an extra step. You have to give GMail your SMTP submission credentials for the sending server of the target domain, then prove ownership of the specific address you want to send as. SPF/DKIM/DMARC works fine, as GMail is just submitting your message on your behalf the way any MUA would.

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throooooo a day ago | parent [-]

Oh, if they're sending via your provider's SMTP then that makes sense. Still, I can't imagine a lot of people use this feature.

	▲	avsteele a day ago \| parent \| next [-]
		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.
	▲	jayturley a day ago \| parent \| prev [-]
		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.