"If only Blizzard had known that lowering the monthly rate from $13 to $8 would make you subscribe for 12 months instead of 2-4 :)"They almost certainly were aware of this, since it's basic macroeconomics that there will be a larger number of people who will buy a product at a lower price point. The tricky part is figuring out when the increased sales to people who would not have paid the higher price are outweighed by the lost revenue from people who were willing to pay the full amount. Perhaps their confidence in retaining subscribers is really low, perhaps they are concerned about the effect that the exodus has on the players who remains, or perhaps Tobold is correct that they hope that free copies of DIII will drive future RMT auction house fees.
All issues of why aside, there's an interesting quirk to this price drop in that it is temporary. Come late October 2012, the rate goes back up to $13 (with the six month commitment). A cynic could argue (Kring did in the comments) that the solution is to charge $60 for the presumptive first DIII expansion and just duplicate the plan out another year on the theory that it worked once. I'm just not convinced that a DIII expansion could be ready in time to fact into a purchasing decision in October 2012 (when it will be likely be just announced, trending towards a Q3 2013 launch). Also, won't they want to actually be paid for their work on DIII at some point?
Will we see an actual cut to the multi-month fee? Some other combination of incentives? Is the plan to prop this thing up until it can be incorporated into the hypothetical Blizzard all-access pass with Titan? Whatever happens, it will be interesting to watch.