BAEC only works if flying WT+ or higher, and the cheapest WT+ fare is generally triple the cheapest economy fare. So it triples their total airfare expenditure. It might make sense to do one WT+ trip, crediting to AA on a Plat challenge to get Sapphire, and then use cheap economy fares (which earn well on CX Asiamiles). You cannot even join BAEC if flying a discount economy fare! BAEC requires a premium fare flight sector before you can get the account activated and start earning miles at all.