The DRM might piss some people off but isn't the server-client integrity quite important when dealing with real money transactions within the game such as the real money auction house?
It would. And it is a problem easily ('easily' in theory, though not perhaps in execution) solved:
I can create characters 'online,' which are stored on Blizzard servers, only playable when connected to the internet for communication with said servers, and can vend their inventories on an RMT market.
I can also create characters 'offline,' which can be played anytime, connected or not, and whose inventories can't interact with an RMT market.
This is a solution which would give Blizzard the ability to keep a tight reign on RMT market activity and still give people to chance to play on a plane, or on a laptop with the wireless off to save battery, or on a dodgy internet connection. It would not, however, serve as DRM, which is half the reason (if not more) why they're doing this in the first place.
Yes, Blizzard wants to control RMT stuff tightly, which is why the game is "always online." But they also want to try and stop the pirates, which is why there's no offline play. Simply put, their desire to try and stop the pirates is more important to them than an offline mode for the (who knows how many) players that want/need it.