There's also the advantage of them taking care of the business side. Reading some of his recent updates seems to indicate that the past few weeks have been spent dealing with administrative duties like lawyers, servers, and hiring. Were he to take them up on their offer, that could all be taken care of for him, if we wanted (I'm guessing).

I'm happy that he's decided to stay independent, but I think Valve could actually handle a hire such as him correctly. They've got a great reputation with most of the gaming community for doing things right, and I'd be surprised if they screwed something like that up.