I'm the other way around: I bought it for the singleplayer campaign. It was terrible: short, half-assed, and boring. Huge contrast to the WoL campaign.
SC2 wasn't really designed for the single player campaign. Neither was brood war, Warcraft 1, 2, or 3. This genre of games has always been about the multiplayer aspect. The campaigns are nice add ons but not the core of the games.
I think you may be a tad misinformed. Even though the multiplayer aspect is very popular, Blizzard as a company has been known for really caring about the underlying story in both franchises. So no, the campaigns - which are the main tool used to advance the stories - are not supposed to be "nice add-ons."
