Originally Posted by lkar

People like US miles because US has a good awards chart and can be reliably used for good redemptions for star alliance flights. There is nothing really equivalent that you can transfer your Membership Rewards miles to if you want to book on *A. Aeroplan is a good option, particularly because it permits one-way awards for less than full round trip, but they now impose taxes. ANA and Singapore are also options, but they have some downsides -- I think ANA requires you actually to transfer miles before you can check for availability, for example. Accordingly, people who like to redeem for *A travel will generally prefer US. When UA was an Amex transfer partner, it was easier, but now occasions to get near 1:1 value to US for Amex is rare.