I signed up for AA and United dining for miles. I split my credit cards amongst them so that my dw and I could 'secure' the miles that we have in their frequent flyer clubs. I have found that we fly whoever is cheapest and have come close to having miles expire, so this is a way to keep these safe. All you have to do is eat out once at the participating restaurants (a lot in a big city like Chicago) every year to 18 months to add some miles to your account and avoid expiration of miles. If you eat out a lot, you may actually get some substantial amounts of miles.
Both AA and United programs are run by the same people, so you have to split up your credit cards to get miles (i.e., you can't use the same card for both programs).
Miles just show up, no muss, no fuss.
Life is GREAT!