One-use codes make me very nervous; either they can be invisibly stolen if they are not tamper-evident, or they "damage" the value of the book to a retailer if they are tamper-evident, as the tampered book will lose value to many (not all) customers. And, of course, the effect on the secondary market is bad as well.
On the other hand, WotC can use individual codes as "market-research". Plus, if they don't make the codes one-use, they get more money from people who would otherwise pirate the IP, along with the people who buy on the secondary market (who would otherwise give no money back to WotC).
(I'm NOT trying to equate IP pirates with purchasers of used books, rather the opposite).
I also don't expect this issue to be clarified any time soon - it's a business decision that may not have been made yet; plus it can be changed from one-use to multiple-use, but not vice versa...