Actually, it's not pointless at all. The RA system assumes that all routers capable of announcing RAs are default routers and that virtually all routers are created equal (yes, you have high/medium/low, but, really, since you have to use high for everything in any reasonable deployment...)
No it doesn't. You can set the router lifetime to zero, which indicates to the end-node that the RA isn't announcing a default router. In this case, it may be announcing M/O bit, prefix or other parameters.
DHCPv6 can selectively give different information to different hosts on the same wire segment. RA cannot.
There are real environments where it's desirable to have a way to tell different clients on a network to use different default gateways or default gateway sets.
Owen