Putting the routing decisions in the hands of the servers(that we do not control) requires that we somehow impart this routing policy on our customers, make them keep it up to date when we change things, and somehow enforce that they don't break the policy.
The same problems exist, on a smaller level, on enterprise networks. Routing policies can be complex, requiring information that isn't currently visible to end hosts, that changes frequently, and can be very costly if anyone ignores the policy. Under current BGP-style decisions-at-the-edges networking, it's impossible for an end user or server to ignore routing policy.
Clearly, it would be extremely unwise for an ISP or an enterprise to rely on shim6 for multihoming. Fortunately they won't have to do this because the BGP multihoming option will be available.
I just don't think it's a solution for everyone.
It may never be a solution for anyone. --Michael Dillon