I dont know about BKK, but they certainly do make sure the priority boarders are way ahead or have already boarded before allowing the remaining passengers to proceed to the aircraft in airports such as KUL and HKT.

Having said that I agree its a complete waste of money. You are not guaranteed any particular seat on the aircraft and on some flights I have seen 20-30 passengers opting for priority boarding when there are only a handful of "premium" seats.

Even if you manage to snag a nice seat, someone will always decide to sit next to you (in the middle seat). From my observations european passengers on Air Asia tend not to understand the concept of "spreading-out"; I often meet passengers who insist on sitting next to me when the load factor is less than 60% meaning no one has to take the middle seat