Valve doesn't have nearly enough of a userbase for their own games... Blizzard has about 11 million WoW users already, and I'd say with Starcraft and Diablo it will add up to about 20-25 million at least. This is a market that could be worth to try something unique with it.
Steam as a distribution platform is the coolest thing ever, obviously, but I don't get this feeling that they could do more with it.
Blizzard on the other hand is making active steps in separating their user base away from the rest of the PC gaming world IMHO. Which also means closing out the competition. Linking every one of them together with Battle.net - which they make obligatory in SC2 - is a very big and bold move. And I wouldn't be surprised at all to learn that D3 will be the same. Notice they only mention user benefits at Blizzcon but it obviously isn't there just for the gamers.
This way they can have a stronger presence and extend it to their other games and services.
Realize that people will probably chat less and less using ICQ/MSN when they have a B.net version for their online gaming pals... And seeing others' achievments in games you don't yet have is also a huge motivational force to buy that particular game. They're still building the groundwork but probably have other plans for the future as well.
So all in all I feel like Blizzard is building a closed gaming platform for themselves. It's software only on the client side at the moment... But they already control the hardware on the server side and so it would be a natural next step to extend their reach to the actual gaming machines too. Imagine all those Korean net cafes buying Blizzard gaming systems by the dozens or even hundreds. And it wouldn't be just about revenue, they could make sure to include hardware features they could make further use of.
But Valve isn't interested in locking out other games because they get revenue from that, so Steam won't become any more closed then it is now (pretty much everyone's free to join and add their games). Their own game development is probably migrating to the X360 anyway and they're probably designing HL3 with a controller in mind.
Also note that I'm not saying that Blizzard's evil or anything. It's natural that they try to capitalize on their supersized presence in the PC gaming market.