I really don't understand your angle. A person dumb with money is going to be stupid with it regardless of source. There is a measurable benefit to using a card responsibly.
You're working under the assumption that a person's income covers all their necessary monthly expenses and only uses the card to buy useless stuff?
I don't get it either, I fly for free with the Southwest Companion Pass simply by buying everything with my wife's Southwest credit card. It's stuff we buy anyway, and it's paid off every month. Never a penny of interest paid.
I would definitely call it a benefit.