I don't completely agree with the Halo:CE argument so much because, one thing is missing. It's not 2001 anymore. At the time when Halo came out, the concept of online console play was very limited, but the idea of local multiplayer was still very current. The more that online became popular, the more things had to change.
Have you ever played CoD 3, UT3, TF2, with less then four players? It's terrible. The reason is because the maps are not designed for that few of people. So if The Conduit wanted to do local multi-player, they would have had to thrown extra maps in there, possibly pushing things back. Also, there is not much need for local multiplayer anymore in the FPS genre. Co-Op could have been great, but how much would it effect the mechanics of the game?
With that said, I don't think the Conduit's sales will suffer because of it. I think it will suffer because of it's platform, the style is a bit lacking, and their marketing has been there presence wise, but not done well. I have the vibe that High Voltage does not have a very big PR team, and have to resort to directors/developers to do the talking of their games.