You actually have some evidence that most purchases are from one-time use wallets?
Also, the money has to go into the wallet somehow. This would mean they would buy the exact amount of bitcoins they needed for that particular transaction and that then goes into the wallet they intend to use for that transaction and then delete the wallet. I think that is too short-term for most bitcoin users. Who in their right mind would buy bitcoins day by day as needed? When the value fluctuates so wildly?
I think its more likely they have a bunch of hoarded bitcoins (that they bought when there was a dip in price), maybe transfer some into a one-use wallet and then use that wallet for the dodgy purpose, being unaware that the wallet this bitcoin was transferred *from* is easily determined.
The potential for mischievous use of the block chain is awesome. Could be a marketers wet dream. Especially as less tech-savvy people start using bitcoins.