There is no requirement to have a Platinum card to qualify for Centurion. A Gold or Green card will work just as well, and with the Premier Rewards Gold card you are likely to earn more Membership Rewards points for the same amount of spending. If you do not fly AA, CO or DL often enough to recoup the Platinum card annual fee through airport lounge access it's difficult to justify holding.

Delta is not your only option. You could redeem Membership Rewards points through Aeroplan, ANA or Continental for travel on US Airways. However, this approach does not allow you to combine miles from flying with miles from spending unless you also begin to credit your US Airways flights to one of those programs which is typically not attractive for obtaining elite benefits.

On the other hand, much of the benefit of Membership Rewards is the ability to accumulate points to transfer to a program that you otherwise do not use, for travel on an airline that you otherwise do not fly. Through Membership Rewards you can travel on any member of OneWorld, Skyteam and Star Alliance.