Camryn Manheim has a new show for next season, Extant, so I suspect we won't see her much, or at all, on Person of Interest nest season.
If the show runners don't get rid of Root, they will lose a lot of their audience. She is not at all entertaining to watch; her use of the female pronoun when referring to the machine is like scraping chalk on a blackboard to me. I generally use the time her scenes are on for a bathroom break or to make a snack.
Greer is an example of a villainous character who is consistently watchable. His performance just draws the viewer in. To me he seems to be a 21st century Adolf Hitler whose motivation is to control the world, run it as he sees fit, and to exterminate his "undesirables."