I think it would be quite handy to have a module & scanner for a local search too.
I would create a combined search for the web and local documents together.
An example might show the benefits of this approach.
Today I had to find out some advanced information about a special barcode workflow to be implemented. I was sure I would find some relevant information in my file system , some in my mails, some in some DevonThink DBs and some in the web. All together in one DevonAgent search would be a lot easier to handle.
This setup could be improved further if there would be a scanner which compares the URLs of the DT or filesystem docs and compares these to the results from other scan modules and shows equivalence or differences.