Well.. if you use a credit card with a fee that's lower than 1.7% (or however many pennies you value your miles at), then it's reasonable. You probably can't go any lower than 1.25% on fees, so you'd only earn under 0.5 pennies per dollar spent.

Still, if you can pay yourself enough money to make up for the fees (at least $3,000/month to cover $15 + some fraction of the fixed cost of the app ($50)), then it's free money.

So, on one side, you can keep carrying heavy coins, giving yourself back pains, and wasting time going to the bank and dealing with annoying managers/tellers. On the other side, you can sit on your ... and charge your credit card some money while watching tv. You can even set up automatic payments and have a constant influx of miles with not work at all. But you'll earn miles more slowly, in an arguably unethical way, and bear the risk of exploring new territory.

Who wants to try this out?