They have certain similarity that if we are going to solve mobility issue in a way different from MIPv6, which is hopeless, minor details of M6 design will be affected.
My opinion, for what it's worth, is that we're not out to solve the mobility issue in Multi6. I don't think we want to advertise that we will work on mobility, as mobility has out of scope. If we are clever, the solution for multihoming will be useful for mobility. However, I would not like multi6 to get a storm of drafts about mobility when we are chartered to solve multihoming.
Proposals from those who half understand the relationships will be really annoying.
Thus, in my proposal, details of possible mobility solution is dropped, though minimal explanation on why a bit is reserved is given.