Both MPC and BAEC can feed into the same AsiaMiles account no problem.
The main reasons for MPC (Diamond) rather than BAEC (Gold) were:
- access to The Wing even if flying on non-OW airlines
- priority baggage handling (at HKG this is great - never any waiting)
- guaranteed seat on CX at lowest published fare in economy
- allegedly CX has all sorts of invitation only events in HK (but unfortunately, despite being Diamond for the last 3 years they have never seen fit to invite me)
- top priority waitlisting (only one flight in 300+ that they failed to get me on); never fail (yet) to get redemption bookings