Is the nub of the nub of the problem that the client may not know what kind of representation it might get back when dereferencing a URI? If the client does know the type of representation, or if asking for a particular type of representation results in an error from the server, then it can assume to know what the fragment identifiers mean. Right?
While that is true, it's not relevant to this argument. The core of the problem is that hashed URIs are inherently ambiguous. Its meaning depends on how you access it, which is nuts. Its as though a word has different meanings depending on whether you read it in a book or have it read out to you.
Bill de hÃ“ra points to a similar writeup he did a month or so back (which I should have remembered and linked to when I wrote mine!). It contains this classic de hÃ“ra line:
You really don't want your absolute naming system for a planet to be driven by an arbitrary feature of a markup format
Danny weighed in with a long post that darted onto the representation vs description issue about which I also have a bit to say... but in another post :)