The only value in purchasing airmiles is if you are within a nats breadth of having enough miles to 'pay' for a reward flight with miles and just need an extra 1000 or so to get you there. Bulk purchase of airmiles is just grossly uneconomic compared to discounted flight prices.
Heck, I went to Singapore for a long weekend with a mate a couple of years back and we only paid £296 each with Singapore Airlines. I bet a reward flight could well cost more than that!
I'll be interested to see how user friendly my bmi Diamond Club miles are across the Star Alliance network when I try to book a return to Oz in 1st class next year. I've heard that doing some leg work (for options) yourself sometimes aids when dealing with the less than enthusiastic staff on the phone.