But when I can buy a roundtrip ticket for HK$1290 (any day that I can see (min/max 2/7 nights, no miles)) and business class for HK$6560 with a total cost of HK$9140, why would I spend HK$9900 for this "special offer"? Pay HK$760 more to gain 3116 AsiaMiles?

That really doesn't seem very special to me.