I find it somewhat hilarious that some people here are siding with the publisher (or at least seem to be). I'm assuming it is more a factor of they want the game without having to use Steam. That's cool. As long as Valve provides this somehow, then I'm assuming you would be happy. Heck if you want updates/patches for the game, you can even go to one of the CD Burning services (think GameSpot has one) and just get your updates burnt and shipped to you (at cost though) or just get them off a friend. There are ways of getting and playing games without having an internet connection.
Still I think the more important issue here is ownership. Assuming Valve hasn't broken their contract with Sierra (no idea where you can find these details), then they should have the right to utilize Steam as their own alternative distribution method and make money from it. In otherwords, they sweated making the game, therefore, shouldn't they be making the profits from it. Yes, Sierra gets distribution rights for physical CD's but Valve should still be able to still make money from their Steam distribution system. I mean this is a long time coming so I can't see how Valve wouldn't have worded their agreement with Sierra appropriatedly to take this in account.
Finally, the main reason that other developers can't take the same path as Valve is because they don't have the money to make the game in the first place (whereas Valve apparently does have their own piggy bank). These other developers need publishers to fund them to help them produce the game and publish it. However, if Valve's system works for them, I can't see why they wouldn't over time (with it being fully tested and proven) that they would offer the same service (or license to use the technology) to other game developers so that they could benefit more directly from their games as well. Of course, I can't see publishers allowing developers to go this path especially if the publisher is footing the bill for the development of the game. However, who knows what the future holds for game development funding.
Still all said and done, if Sierra forced ownership of the game via the courts and didn't allow Valve to utilize steam as an alternative distribution method and more importantly if Sierra screwed Valve with regards to future HL franchise products, well I'd be immediately boycotting Sierra as quick as you can say Gordie. I'm sure people would think I'm nuts but hell I've had enough with greedy corps, thank you very much. The Internet has the opportunity to change a lot of things and I for one am glad it is allowing creative individuals more control and ownership of their creations.