Is this why back in the day sometimes a Linux distro would have a multi-monitor setup where each monitor was an actual different desktop cube for example. There was a time when each window for an Nvidia graphics card in that type of configuration could not be moved from one screen to another, etc.
Given that they're fine with adding breaking changes to the protocol, I think it's a shame that they're not supporting multi-screen. This will lead to the same problem with Xrandr based multi-monitor where you get screen tearing with mixed refresh rate displays. I would prefer to see "traditional X11 multihead but you can move programs between screens" as the solution for multiple monitors. Even if it worked like Mac OS X where you can't have a single window span across multiple monitors, it would still be better than the current state of X11.
In X11 "screen" has a particular meaning, and only supporting a single screen doesn't preclude multi-monitor support or virtual desktops.