My own thoughts are if any console developer really wants to make a splash, they will make games that are playable, online, with the PC version.
Games are all about playing what other people are playing and gaining bragging rights.
There was a reason Microsoft bought Bungie Software. That reason, as we all know, was Halo . Every console needs a launch title and Halo is perfect. It would be foolish if Microsoft were to compromise on their consoles visibility by releasing Halo simultaneously on the PC and the Xbox. In a way, yes, but Halo is not the rule, it is a very special exception.