To be honest, my anticipation is actually greater due to the fact that it's a standalone product. If the addition were a patch (much like the addition of LANs with Nakatomi) one wouldn't expect them to do much more than provide multiplayer support (not to imply that IV does anything in halves), but in knowing that it's standalone I also know that it'll have the content to justify the move.
You note "when I buy a game, it usually has a multiplayer component free", did you consider that most of these games are also over twice the price of Darwinia? If I were you I'd be praising how generously IV have priced Darwinia already. Indeed, your argument contradicts itself - you argue that most games you buy have a multiplayer component free, and then that Valve didn't 'have to' release HL2:DM.
I heartily disagree with your assertion that it's a 'cash in': while IV is no longer limping along as it was during the development of Darwinia, as an independent company they need all the money they can get. The reality is that first and foremost, the business needs to remain sustainable, and while if they were making a few hasty changes and booting the product out the door one might protest, this is a considerable project that is being undertaken, as evidenced by the expected release date.
Nevertheless, it would be good to have the question of single-player inclusion cleared up. The press release notes "as well as the traditional single player mode..." - does this consist of the original single player levels, or new levels which can demonstrate the new features etc. implemented for Multiwinia?