gamescope is the bees knees when it comes to older games with strange resolutions, ultra wide monitors or games that don't behave well with alt-tab. For example the old You don't know Jack games render in unscaled 640x480, even on windows, which make them unplayable on a modern displays. But with gamescope that isn't an issue.
So why not make every linux users life easier by integrating gamescope directly into the steam client. It is developed by one of the valve devs anyways.