I don't think it directly duplicates theming. Most of
the work for theming will be cleaning up existing markup
so that it is less presumptious about layout (make it all
semantic with suitable css classes), improving documentation,
and breaking some things out into includable templates.
In fact what you are talking about is quite complimentary
to theming, although I think you will need to be quite
thoughtful about how you support different subsites using the
same templates (and it would be nice to think about how to
export those templates to others as well).
