I'm of two minds on the "team decision" thing:
1) As the media would like us to believe, Rex is basically a lame duck being forced by Idzik to start his prized draft pick.
2) This is truly a team decision, between Rex, Marty, and Idzik. Geno is who all three view as the future, and all are willing to take whatever growing pains come with continuing to leave him under center.
My leaning is the latter. Between Marty's quotes regarding growing pains, Rex's brutal honesty, and Idzik's repeated insistence that Rex and him are on the same page, I find it hard to believe that the entire management of the organization isn't fully on board with Geno and on the same page.