> except in ipv4 getting a link-local address means "I fucked up DHCP" […]

No, it means "there is no infrastructure on this link segment". No router (to send out IPv6 RAs), and (as you say) no (working) DHCP server.

Still being able to have network connectivity automatically in this scenario can still be handy. If mDNS is running on things, then the user doesn't even have to bother manually setting an address: the link light comes on and you have connectivity to the local segment.