American Express Membership Rewards points convert 1:1 to miles on all of the carriers listed here (the conversion rate is different for airline programs which do not use "miles" as their loyalty unit):
http://www.membershiprewards.com/cat...lFreqFlyerProg
You are correct that AA does not participate in Membership Rewards, but MR points can be used to travel on AA by redeeming through British Airways Executive Club (currently excluding transatlantic flights between the USA and the UK).
Most airlines belong to one of three aliances: OneWorld, SkyTeam or Star. In general, it is possible to redeem through one airline for travel on any other airline which belongs to the same alliance. This means you do not need to be able to transfer to the airline that you want to fly, only to one of their partners. However, alliances do
not allow you to pool miles from different frequent flyer accounts. This means you need to plan ahead to accumulate miles in only one program per alliance.
The benefit of using a Membership Rewards card versus a card tied to one airline is the greater redemption flexibility.
You have made a good point about about redemption flexibility, which is why I will use the card for certain purchases. However I would not use it if I intended to convert them to a different carrier with the intention of using them on AA (oneworld alliance), since I don't know how easy it would be to mix and match miles from two different carriers.