When I had a PA, I used her to streamline things. She'd see the kind of things that really didn't need me, as mentioned above: tube checks, allergy visits, routine post ops. For certain new patient visits, after sufficient training, I'd even have her book cases (ear tubes for standard indications, tonsillectomies for standard indications, etc.) She'd forward on to me anything more complicated, which we tried not to book in to her clinic in the first place, but sometimes that stuff slips through. I never used her in the OR, because she didn't want to do that. If she had, I might have used her on occasion, but frankly I always felt that part of the benefit was that she could cover clinic issues while I was operating.
Currently I don't have a mid-level, just other MD partners. And I agree that is less ideal. There's more feudalism and if they don't do certain cases that I do, they don't feel comfortable seeing patients after those surgeries, whereas I can train a PA to know how to deal with it.