Rolling expiration is a negative feature, as is automatic issuance of awards that expire in 12 months. (JetBlue's TrueBlue has an even harsher 12-month rolling expiration.) The solution for Rapid Rewards is to get their credit card for $59 and 16 free credits with one of the seemingly constant promotions. Churn it annually if you like. The 16 new credits rescue any about to expire credits, turning them into an award.

For kids' accounts it's tougher. I use Amex Membership Rewards to top off those accounts with the precise number of credits needed to avoid having credits expire.

All FF programs require a certain amount of gamesmanship for best results. If this were not the case the programs would be less rewarding, as you say.