As it is very clearly stated in the HIP WG proposal, the purpose of the WG is to complete work on HIP in order to allow wide scale expermentation. In order to succeed with this goal, there must be a stable understanding on what HIP is. Publishing the architecture draft is an attempt to create such a starting point.
What I am fearing is an architectural battle within a working group. We have seen such working groups -- they seem to take very long without any concrete results. I don't want the HIP WG to be such a WG.
Hence, the motivation is to provide an exact but flexible enough definition on what HIP is, for the purposes of the WG.