Nina Falcone has given up on cash.

Whenever and wherever possible, even at the vending machines in her building in Chicago, the 25-year-old marketer uses her Southwest Airlines Rapid Rewards card to collect points she says she uses for plane tickets to visit her family in California.

Ms. Falcone carefully follows the advice from consumer advocates and does not carry a balance from month to month or pay humongous interest charges.

But she admits there are probably some downsides to the ease of purchasing. Time magazines piled up around her apartment and gathered dust after she bought a subscription simply because it came with an offer for extra points. And she has increased the amount of time she spends shopping on the Internet because merchants offer incentives online for cardholders that are not available in stores.

“I haven’t paid for a trip on Southwest in years,” says Ms. Falcone, which may be technically true, but a host of economic and social science research suggests that consumers tend to spend more using plastic than they ever would with actual cash.