Steam was used during beta development for the ease of pushing updates, afaik. I am also disappointed about the lack of GOG, but you can always grab the game and then delete steam from your system. The game itself is still DRM free, even if it is delivered through a DRM system.
One of the unfortunate side effects of making it hard for people to access media - is that they might just access it anyway, without any of the hassle.
Anyway, I really dislike how on steam I cannot see the Source Hunter pack on my games list, even though I bought it and I have the trunks and goblet in game. Not sure where the artbook, dev speak and soundtrack will show up.
