Expired miles
 

Why do some FFP programs have expired miles, some don't? To member's benefits, what is pro and con?

Sorry, but maybe I did not get your question as I see no benefits for me if miles expires.
In any way:Don’t bank your miles!

There are no benefits to the consumer from miles expiring. Airlines, on the other hand, view it as a plus -- an expired mile doesn't cost them anything.

I'm not sure the real equation is that simple though. Expiration encourages faster redemption, which costs airlines money, and if given the choice an FF will opt for a non-expiring FFP (and airline) instead.