Milkmood. I think a basic answer to this question would be that a properly designed permaculture landscape IS quite aesthetic in appearance. And the design is supposed to permanent with every plant contributing to the whole.
So if you are talking about having a plant that makes no contribution I would ask why do you want to put it in the context of a permaculture landscape?
I am thinking perhaps of the Knockout roses. Why use them when natural roses attract wildlife, have edible rose hips, they bloom in season instead of boringly all the time?
But then what ever one finds "aesthetic" is highly personal.
