Sure, THIS time he is paying for a largely multiplayer product, but what about next time he pirates? He won't pay!
So what your opinion is doing is O.King the constriction of the P.C gaming industry. I say, no tolerance!
I'll tell you this: pirates generally only buy games which they feel are worth their money. That is, games with a fun multiplayer component. It is a huge selling point, even for people who don't pirate, and it is one of the only things that will make a long-time pirate buy a game.
The multiplayer component adds more replayability than anyone could ever hope to achieve with single player, and that is what really makes the purchase worth it.
OP is simply saying that, if Stardock & GPG offered this as a free download and charged for the multiplayer key, they would effectively be eliminating all piracy, and still maintain the main (and only?) selling point of the game. They would also expose the game to a large audience of anti-piracy zealots who refuse to pirate to try, but are still on the fence about this game, so they choose not to buy it.
You will find that many pirates, much like the OP, pirated the game because they were on the fence, and then promptly decided to buy it because they thought it was worth it. The same will happen with what OP suggests, only on a much larger scale.
Indeed there are pirates who refuse to buy a game, but there's also a large amount of pirates who are just out there to try a game. The economically conscious pirate, if you will. See, these are the same people which will do extensive research on a TV before making the purchase. Many of the people who just see the game and decide to buy it are the impulse buyers which look at the screen size, the brand and the screen tech (TFT, IPS, VA) (the graphics are nice, the idea is nice, GPG is good) and buy the TV.
"Don't judge a book by its cover." - Pirate-to-try pirates want to read the first few pages before buying the book, but you just look at the title, the author and the cover picture and purchase it. It is a difference in personality, not ideology.