The ability to manually specify the Link-local address can make other things easy than just a consistent default GW on all network access segments.
well, you can still specify addresses if you want, as you're not limited to have just one or two addresses on the link.
but ipv6 is about autoconfiguration, and it relies on link local addresses for everything. once this address configuration mechanism is screwed up, nothing will work. usually there is no reason to have the same mac addresses on both sides of the link, hence the generated ipv6 address is always unique, unless you tamper with it.
i'm not aware of any other non-ethernet L2 media that supports multiple access, and in general, with P2P interfaces you (almost) don't need addressing at all, you need routes pointing on the P2P link.
routers are pretty good at figuring out LL addresses for the other parties. you can use OSPF to have this sorted out easily and you don't need GUA just for your loopback and customer facing links, maybe. servers (or generally non-router hosts) will be just fine with RAs to pick their default gateway(s).