I'd use it because it's just easier. You can preorder right through Steam, have it preloaded and ready to go when it gets released. Plus I already have a bunch of other games through it, so it's just convenient that I have most of my games in one spot. And I have a bunch of friends with Steam, which allows me to talk to them through the Steam client without having to alt tab and they can be in different games. I like Steam quite a bit, I wish more companies used it. It's nice not having to worry about misplacing CD keys and keeping track of everything. And for me at least, it works pretty flawlessly and is much easier to get updates for games. I wouldn't really personally worry about Steam going down. I tried the trial for EVE through steam, and you can actually run the game outside of Steam.

And no matter if Steam is great or not, it's still more exposure for the game which will mean more sales. It makes sense regardless if you see it or not.