jamyskis: And besides, I think you're overestimating the size of the "no Steam, no buy" crowd. Vocal as they may be, there are very few people that refuse to use anything but Steam, and fewer still who refuse to buy anywhere except Steam. Most Steam-only users also shop around and buy from Gamersgate, GMG, Humble Store etc.
I have several things to say about this. First of all you're right in concept, "no Steam, no buy" simply means that the game must be activated on Steam, not that the game must be bought on Steam. Such people will buy on other stores or bundles, but only if they get a Steam key.
Secondly, regardless of what you think of people and where they buy, Steam is the largest digital store, and a lot of people buy existing games which have been available elsewhere when they appear on Steam. These sales tend to dominate the game's sales.
As I said before, many people are risk averse, and won't buy before there's an actual game. While this will only be half a game, it will probably be enough to convince many people who haven't been willing to play before even if they heard of the game, and it will draw in some people who haven't heard of the game before.