Steam is an actual platform that does stuff. Humble Bundle is pretty much a website with some download links. I do like what Humble is doing though.
My point about Valve and Steam is that there is a lot of potential in that market segment, and certainly Valve hasn't exhausted all of it. Indeed, one could imagine the Humble Bundle working with Valve to get included in Steam sales.
Humble Bundle has a brand people recognize, like, and trust. A hell of a lot of industry connections. And an established track record. I don't know the depth of the gaming industry well enough to know exactly how wise this investment move is, but it seems like it could be fairly reasonable to me.
3. Profit! 4. Raise VC Money 5. ???
I also smell a partnership with a big gun (like Xbox Live Arcade)