I would say for 90% of regular flyers, lifetime MM benefits will not apply. If you are a heavy flyer (100K+ miles a year), you should be achieving top elite status on most airline and in less than 10 years, you will achieve MM status. For those who are actively top tier elite status flyers, MM doesn't mean much except for bragging rights or in the case of United, getting your spouse the same elite status. It becomes more helpful when your travel pattern changes where you don't fly as often and the airline will still provide decent benefits to you. However with many people being able to get "elite" benefits by paying for them or owning a credit card, I don't know if these benefits are really as valuable as they once were.