Multiplayer is where it's at, I don't see much wrong with pirating it just to play the single player campaign to be honest. If you buy the game just to play the single player campiagn then you really do not get your moneys worth.
I agree, it's not like Blizzard spent any time or money on the single player game with any thought of possibly recouping that investment through game sales. There's also nothing as fulfilling as spending years of your life working on a project only to have some self-entitled piece of shit tell you 'meh, it's not worth sixty of my dollars...but I'll play it for free!'
They'll always have the satisfaction of knowing that people can entertain themselves with the game they didn't pay for while sprinkling negative reviews amongst game site forums about how it's not really worth the money they didn't spend.
FYI, I bought the $99 collector's edition with very little interest in playing multiplayer.