It is currently not possible to do source address based routing without firewalls such as pf or ipfw. I think that in typical multihoming scenarios (i.e. mobile devices) it doesn't make sense to have a static firewall configuration file or have some daemon dynamically mess with my firewall rules. I suggest making it possible to configure one routing table per address instead of only one per interface with ifconfig. This probably would make automatic multihoming configuration with RA or DHCPv6 easy to implement. Linux solves this problem the same way.
For Multipath TCP, proper multihoming support will become necessary.