Trilarion: I think the two things (can be sold where-ever, can be played where-ever) aren't independent from each and both influence the possible competition. After all, selling Steam-exclusives through other shops is just selling a serial number. The profit margin for such a thing cannot be very high and there competition only has a small effect. But if Steamworks (and all the systems of the competitors) would be open and usable by competitors the prices would probably drop.
Adding Steamworks comes at no cost to the developer as far as I'm aware, so I'm not sure the developer/publisher incurs any extra cost selling it's Steamworks game through other portals except what those other portals ask for selling it. I'm not sure it cuts into their margin in that sense. (but I could be wrong, but not aware of the contrary at the moment).
I already admitted it probably affects the competition, I disagree it's closed though, which I don't think it is. If it's closed in any way, it's because the competition can't offer the same service (Steamworks) that Steam can, else the developer could simply make their game without Steamworks and sell it at the competition, of which there is still plenty. In that sense, you could argue Steam is in a dominant position, but considering that is because they are actually offering something that helps developers make better games I feel it's up to the competition to come up to speed on that, not for Steam to lay off on it.
I haven't been able to find it for the life of me but I'm pretty sure I remember reading somewhere that even games made with Steamworks don't *have* to be linked to 'need Steam to play', but I can't find the post anymore, and I am not sure that if it were the case if that's still true, or perhaps they're offering other benefits to those people that tempt them to make it 'steam exclusive' anyway. (less profit margin or something perhaps)