The "multi-release" is no different than SW/BW or WC3/TFT. The only difference is that this time around there's a common multiplayer platform, meaning that those folks who, inexplicably, aren't interested in singleplayer don't have to purchase the subsequent expansions to receive the multiplayer updates contained therein.
The singleplayer/multiplayer divide in
Starcraft is interesting of itself. SC's legacy is primarily a multiplayer one; it's the depth, balance and general appeal of its multiplayer component that's sustained the game over the past decade. Yet it's not the whole story. Upon release
Starcraft was equally as lauded for its singleplayer as for its multiplayer, with Gamespot going so far as to award it a "Special Achievement in Interactive Storytelling". I forget the exact stat, but the
majority of folks who purchased
Starcraft never went online with it.
Starcraft's multiplayer is the reason why folks still talk about the game a decade past release, but it's not the reason it was widely considered - along with
Half-Life - the best game of 1998.
Fast forward to SC2, and the conventional account is that it's the same game with only the barest concessions to modernity. Yet this narrative is derived entirely from the game's multiplayer component - which, per above, can hardly be said to represent the whole story. There is no singleplayer beta, of course, but from the information
released it's there the most significant changes to the game can be found. Ironically, the only attention which the singleplayer component
has received has been negative, i.e. the different campaign structure.