Originally Posted by number_6
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.
BAEC does allow earning in the following economy (WT) classes: Y, B & H. The first 'premium' fare flight you take will allow you to open the account and simultenously open and credit the flight to your BAEC membership. You can get around this by applying for a BA Amex credit card which will open a BAEC membership for you without requiring a F, A, J, C, D, I, W, T, Y, B or H class fare. Fares below the above earn only 25% miles and don't qualify for tier points.
I do however agree with number_6 that maybe AA for a single WTP fare maybe worthwhile doing a Platinum challenge on AA, then credit everything to Asia Miles. Then again, if lounge access is the only requirement, just put everything to Asia Miles and get a Priority Pass membership to get the lounge access.