When I share something to Plus, now it shows up to people who are on Plus. They are not in "let's read my RSS feeds" mode, they're in "let's see what my friends are up to" mode. My shared article, instead of being just another item to read and potentially comment on, has intruded on their social activity. If they wanted to read about quantum tunneling, they would have gone to Reader, not to Plus. Plus is where you go to read about your cousin's new haircut.
You're describing a distinction that exists on a service level now, that's basically arbitrary and inconsistent. With everything pushed to Plus and pulled back out of Plus, you can define not only social contexts like 'friends,' 'co-workers,' and 'family,' but also subject matter like 'souffles' and 'quantum tunneling.' This is a good thing.
N.B. I think this will only be truly useful if they add public circles as I describe earlier.
I guess I'll share certain articles to specific circles, if I share them from reader at all.