I am not sure these developers would come onboard with knowledge of javascript - so the biggest advantage (ubiquity) is nullified.
I am not sure which developers is Gnome targeting by choosing javascript. At this point in the evolution of OSes, its all about the app marketplace.
By what numbers? Number of projects using the language? Number of lines of code in the wild in that language? Number of users using apps written in the language? Because I don't have the numbers in front of me, but I seriously doubt that that's correct. Firefox, Chrome, iTunes, Photoshop, Dreamweaver, Excel, Word, Powerpoint... None of these apps are written in C#. The most popular app dev language, period, is C++. (Maker help us all.)
How JavaScript fits in this picture, however, is still beyond me.
I'm finding it hard to believe that you can woo quality desktop app developers using javascript.
Otherwise, I'd call the Free Software Movement a pretty neat ecosystem :P I'm sure at least some people will agree :)