A host-based, host-only multihoming solution does not give the network the
visibility into alternate paths to a destination, and therefore cannot
apply policy to it.
The only point of control a network operator would have in that
environment is some kind of policy mechanism on hosts. I don't see it
scaling to hundreds of thousands of nodes unless the central policy point
is a function of the network.
I don't think there is any requirement that a host make its decisions about
address selection _in vacuo_. It makes sense for it to be able to obtain
assistance from a source which is aware of network policy.