It was driven by Ballmer's ego more than a rational plan. They imagined they could speed time to market for devices by eliminating frictions inherent in being

It was driven by Ballmer's ego more than a rational plan. They imagined they could speed time to market for devices by eliminating frictions inherent in being 2 companies working on 1 product (and as an employee I can say there were indeed inescapable frictions). But ultimately it couldn't get around the core problem: (almost) nobody wants a Windows Phone.