They probably make their money from people who don't use all the flights they booked. Personally, I find that business model horrible and it's what you get with race-to-the-bottom markets like cellphone plans, electricity and credit cards. It screws over people who misjudge their own future usage but give hope that you'll save money if you get it right.

I suppose it's still a good option to have since customers are effectively being paid to do the work of planning their use in advance which no doubt reduces costs for the airline.

Hi, they don't roll over. But we're working on a feature where users can transfer unused flight credits to family and friends.