Being that Smartglass is a precise clone of Apple Airplay, I think it needs to be interpreted as a response to Apple capabilities in cross-device media sharing.
Airplay is exceedingly simple. There is no prior setup, no sync, no user-visible state transfer, no unnecessary session management, it handles multiple users properly, it backgrounds well on iOS, and authentication is "free" since all that's handled at the Wifi layer.
When you see people in wizzy concept videos flicking content from one device to another, this is what they're doing. The only thing they get wrong in these futuristic videos is that this stuff has been shipping for the last two years on a major platform.
Microsoft was correct to clone this.