I have a friend who is dating someone who has no opinions. She seems to not care about anything. On the one hand, it's great because she's pretty much always up for whatever he suggests. On the other hand, it can be rather dull at times. Friend explained that when he expresses his opinion and asks what she thinks, she pretty much always agrees but doesn't add anything. We decided this is conversational plagiarism. You can't just take whatever I said, simply add, "I agree", and have that count as a meaningful statement.