I purchased off Steam, so cannot verify for sure. But reports suggest the CD is required in the drive to play the game
Presumably this is a condition of the publisher not of Valve, who we can assume told them they didn't need to bother.
I read a comment here about Lou Gossett jnr (who I have a soft spot for after his seminal performance in Predator) so I cracked open my retail version of HL2 (to recap, I preordered but couldn't wait 24 hours to play so I hit Steam on Tuesday evening and was playing inside a couple of hours - impressed was I...) to check the credits in the booklet. There wasn't even a booklet. I don't know if this is universal but my UK standard DVD contains a DVD and a piece of paper with the controls printed on it. Again, to me at any rate, this suggests that nobody at Valve wanted to put together a booklet and the publisher wasn't going to - so no booklet. Now, like the rest of you, I've never read an instruction booklet in my life. Ever. Hardly. And if there's no instruction booklets and you have to keep a disc in the drive and it takes a trip into town to pick it up then why is anybody going to buy games in shops? It would take, perhaps, a 20% saving to sway the market I'd guess - and that's probably more than the publishers take. Curiously I paid more for my HL2 Silver (Steam) than for my std UK retail DVD. It's okay to say that buying via Steam is less than RRP but nobody pays RRP.
But, again, this isn't Valve cutting out the publisher, this is Valve becoming the publisher. And it's only what id were doing 10 years ago anyway (and Apogee and everyone else, and lots of others now for that matter.). Back then it was shareware and you at least got a chunk of the game before ponying up your readies (somebody sell them a dictionary...). I'm still not knocking Valve but obviously there's a bigger picture being painted here than anybody has apparently recognised yet.