Well opening it he would not interrupt the flow and it would be sort of cool to have the main guy hand it off the to the only other main guy He could talk about internal support and then finish with something like, "I think you will see that with the actual presentation and here is Phil to announce CRACKDOWN 2! Ha.
True, but he doesn't have to open it to be at the very beginning. They could still have a few introductory things or whatever before him. I mean, obviously they're going to have the welcome before him if he shows up (and before that probably some clip from some game, like they've done the past few years with Call of Duty or MGS last year), but there could obviously be a little thing before he shows up. Like there could be a quick little "we are committed to games!" speech, and then have nadella come out and talk for a second, and then go back, and then "let's move right on to the games, then, shall we?"