Ars Technica - What EA will lose if Battlefield 3 remains off Steam.
This entire debate can be boiled down to the simple fact that people already use Steam, they like Steam, and they resent being forced to set up an account with another service to play a game. This is especially true when it's a brand-new, untested storefront that's being driven by a company that has given PC gamers every reason to be skeptical. It's unclear if Battlefield 3 will be enough of a draw to make customers forget that fact.

For many gamers, Steam is PC gaming and, while threats of a boycott often seem hollow, in this case it will be as if the game simply doesn't exist for many Steam users.