Robert ... I haven't played with ETP (too busy sliming my way around the guts of things while hacking on non-functional aspects of general-comments and wp-slim) so didn't realize this. That's interesting.
Perhaps packages need to specify whether or not such mounting "makes sense", and the site map UI smart enough to take advantage of this knowledge?
Another "site map hurt" which also ties to package-level information being kept is the distinction between "applications" and "services". The Site Map UI suggests rather strongly that one not map services, even though some (acs-workflow, for instance) do provide web pages for tinkering. I "fixed" this confusion for acs-workflow by auto-mounting it during installation. A more general "fix", which probably involves explicitly declaring whether or not a package includes mountable web pages (rather than assuming "applications" do and "services" don't), is needed.
I'm sure we can add so much to this list that Roger could be kept busy working on Site Map UI issues for months! :)