That’s interesting, if you go back to interviews with GabeN at the time of the Episodes announcements, he was praising Blizzard for the subscription model for WoW. As a company that’s of course the best model to have, as gives more margin (that’s why gyms have very restrictive contracts and so on).
At the moment they don’t seem to have the IPs to do a Valve subscription, but maybe they can come up with a scheme for Steam.