Well, customers who take holidays would have days when they don't transfer any data (assuming they turn off their router/modem while they're away) and customers whose connection is broken because of faulty ISP equipment would also benefit (because there would be financial incentive for the ISPs to fix the problem, rather than it helping their bottom line).
Still doesn't really make sense because the ISP still incurs the same expense. They had to buy that capacity which still exists while you are taking a short holiday. If its a long holiday you can cancel the service for that time.
If your service doesn't work for a significant period, you likely can receive some compensation enforced by a government body.