The short answer is that it is worthwhile joining MileagePlan even though the benefits of the program have been diluted in the last year. The main reason to have a bank of Mileage Plan miles is to get reward upgrades. 5000 miles gets one upgraded one way at point of ticket purchase even on the lowest fare tickets. You cannot do this with any other program that I am aware of.

Should you have some orphan British Air miles, 15000 will get you an AS RT upgrade, also at time of ticket purchase.