I also came to the same conclusion some years ago. Or more specifically, my manager brought me around after I tried arguing that it was worth the time to make sure that users could use an IPv6 address as their domain (the lack of periods after the @ would cause `user@2001:0db8:85a3:0000:0000:8a2e:0370:7334` to fail validation)

He made a very convincing argument that while an IP address is technically a valid domain, but how many legitimate users were seriously using an IP address as their email domain? (zero)