I think we can freely discuss both advantages and disadvantages here.

I hadn't really thought of the miles/point accrual thing since I seldom fly to Asia. To America, you earn the same number of AA miles on AA, AY and BA anyway, and that's where my miles go, so it's not an issue for me. But an important advantage for many, nevertheless.

Seat assignment is what interests me most. If I am booked on AY ticket but BA/AA/IB/JL/xx metal, how and where do I book my seats? And vice versa, if AY metal but AA/BA/xx ticket, how do I get a seat? And what about the costs, operating carrier's price list and policies?