Originally Posted by
aamilesslave
The harmonization of deep discount mileage accrual (25% BA vs. 30% IB) and elite mileage bonus (BA does not participate, IB does). My purely speculative guess is that BA rules will prevail.
And don't forget the harmonization of the ex-US transatlantic exclusion from the AAdvantage program, an even bigger hit (assuming BA prevails) since IB is the (soon to be) only AAdvantage airline with
horizontal lie-flat business class seats you can earn/redeem miles across the pond from/to the US.