Blizzard: Marketing at its finest/most deviant

Blizzcon is what the old Romans used to call Panem et Circenses. Pleasing the crowd with an overdose of entertainment and news on some of the hottest games. They even have their own triumvirate: WoW,  Starcraft,  and Diablo.
It's an also an incredible PR circus and Blizzard are keen masters of suspense, the newest expansion called Mists of Pandaren was probably on any gaming site and blog that is worth a single damn. This one excluded, but then again, it's not really worth a damn anyway.
The most revealing information during Blizcon however, was not the new expansion, it was actually a deal where they offered a free Diablo 3 copy to anyone who took an annual subscription of WoW. You can read it for yourself in full detail here.
It would be naive to interpret this as a present to the loyal fans, and it would not be giving it enough credit to simply say they get their money back from the subscriptions.
Note instead how they give the newest game for free. In the gaming industry this is very rarely done. In fact, tt is usually the other way round. For example how the preorder of Modern Warfare 3 comes with an older instalment of the franchise. Or how at some point you got a free Mass Effect 2 with Dragon Age 2.

This is a marketing move at its finest, and marketing is always a strategic move to address present and future situations. So it is always useful to look at not just what is mentioned, but especially at what is hinted at between the lines.

Boosting Diablo 3 Revenue:

This does not seem to make sense at first sight seeing as how they're giving a free copy away, but let's not forget that Blizzard is making use of an online auction system in Diablo 3. And more precise, an online auction system where they take a cut.
There's also a very likely chance they will introduce expansions to this game as well, because let's face it, Blizzard and Activision know how to milk out their franchise. So having a larger player base will increase their revenue from Diablo a lot further in the pipeline than it probably would otherwise.
So directly and indireclty Blizzard will be getting some pretty decent monetary means back from this deal out of Diablo 3.

Boosting WoW Revenue:

The promotional stunt is of course naturally more inclined to further the revenue of WoW sales, but to understand what Blizzard is gaining here it's necessary to list the effects and background first:

Firstly, this is a very timely deal that is announced roughly two months before the launch of  Star Wars: The Old Republic. TOR is a rival MMO with a gameplay that is almost identical to WoW. Hooking players for one full year to WoW will undoubtedly keep quite a few players to WoW instead of them changing lanes. Of course people will see if the grass will be greener on the other side, but to engage into two monthly subscriptions for two games at the same time, that is a step few will likely take.
Secondly, there will be a guaranteed move from WoW players to the game of Diablo 3. The Diablo franchise may not be an MMO, but it has a deep online cult status. And although both games run on Blizzard's own platform Battle.net, there is still the surface-problem to Blizzard's shareholders that a profit from Diablo 3 could be at the cost of a declining playerbase of WoW. By hooking their own players to WoW, that effect will only have the effect of a mere ripple.
Thirdly, the Mists of Pandaren announcement may have been met with joy, but it may easily cause a rift within the community between the game's huge casual player-base, and it's hardcore and raiding playerbase. The latter is a minority in the game, but it is also a very vocal one. This is further affirmed by many of the game's famous community members foregoing the next expansion for other MMO's or games. Its biggest and most (in)famous example being Total Biscuit.

These are all actions that are specifically aimed at adressing a current and future decreasing amount of subscribers. Before this article was written it has been known that Blizzard has already lost a million subscribers. This is still peanuts compared to the 10 million remaining players of WoW. But taking into account both a theoretical loss to TOR and GW2, a subsequent switch towards D3 and a sceptical reception of its newest expansion means that Blizzard is forced to go on the defence for perhaps the first time in its PR and marketing. That, or they are finally preparing to move their eggs to a new basket: Titan.