I think the role types are mandatory for 2 reasons, one general and one
specific to tm models. The general one is that you may want to make some
assertion between two things but not really know what that assertion is
yet. A real world example would be to transforms someones folder structure
into a tm. Folder structures are weak (non-existant) on what the
relationship is, but the user probably has a good idea about it. Thus
having empty role types and empty assoc type allows for the association to
be refined later. A very useful property when dealing with iterative
topicmap construction. (Personally, I like all my types defined and
organised but appreciate that that isnt everyones world.)
The second reason for leaving types undefined is because that is the
nature of scope. I think we came to some consensus in montreal that scope
is no more than untyped associations between an association and a set of
topics.