Ambivalent characters keep you on your toes as a reader and as a writer. As a reader you are looking for signs that indicate they're becoming a good guy after all (not a chance with Missy!) and as a writer you are looking to put enough doubt into your reader's mind to keep them reading and wondering just what will happen with/to that character in the long run. All thoroughly entertaining stuff and huge fun to write and read about!
Given we are all a mix of good and bad, I think ambivalent characters are probably the most realistic in fiction. Okay now we are having flawed heroes (good thing) and villains who have a better side (makes them more interesting) but ambivalency shows a character doing what he/she/it (am allowing for possibility of ambivalent android here!) needs to do to survive. And that always makes for a good story.