I think the concept of peraculture was intended as a design tool or a model for thinking about our relationship to our constructed plant environment--the gardens we make. The concept focuses on interconnected-ness and usefulness. So a class of plants, i.e. ornamentals, is not necessarily excluded or included. On the other hand permaculture gardens do tend to be food gardens, because that's what we need. There is nothing in permaculture though that says a plant should be excluded just because it is beautiful.
