DELETE vs. UNBIND

A recent proposal is to leave the DELETE method applied to advanced
collection members to be as vaguely specified as it currently is in
the HTTP-1.1 and WebDAV specs, and to define an UNBIND method for
advanced collections with the proposed advanced collection "delete"
semantics.  In last weeks conference call, I agreed to that proposal,
but reserved the option to object to it, if I could think of why it
made me uncomfortable.  Well, I'll now exercise that option (:-).

This proposal has the downside that instead of just issuing a DELETE
request, an advanced-collection aware client must first issue an UNBIND
request, and if that fails because the resource is not a member of an
advanced collection, it then issues a DELETE request.

But I don't see any upside ... a server that supports advanced collections
must support UNBIND semantics, so how does it provide any benefit to
say that an UNBIND request MUST have unbind semantics, while a DELETE
request MAY have unbind semantics?  Perhaps I'm just missing what the
upside is?

Cheers,
Geoff