I learned in Eliot Kimber’s “DITA for Practitioners”, after some re-reading, how the integrator.xml ant script works. It looks for template files and creates the corresponding file from it. When the integrator script creates the corresponding xml file from the template, it inserts “stuff” (different for Ant files or SXL files) from all the places that add to that extension point.
As far as I can tell, and this is not explicit in any documentation I have seen, including Kimber’s book, you only need to create a template file in your new plugin when you have defined a toolkit extension point in it. For plugins that you are sharing, you probably want to add extension points to your plugin. For plugins only used in your own DITA deployment, you probably don’t need to do that.