I agree with you here slim. A reply engages you directly with the OP. A quote will engage OP with the quotes audience. Yes it also garners additional attention but engagement and attention tend to go hand in hand. 

They're not mutually exclusive and the intent is what matters. Is the quoter simply trying to get cheap attention off the backs of someone else's note or are they facilitating engagement with a new audience? Both can utilize the same action but with different intents.