The other crucial argument, which was made in other comments, is that the advantage of JS is further multiplied by its cross-platform nature. Basically every device that matters supports it. I believe that makes the comparison to KDE4 slightly less relevant :)
I have to disagree with this. Creating a form which displays a list of users and another to edit their details is trivial in, say, .NET's winforms. With [PHP,Python,Whatever]/HTML/JS it's a comparatively painful exercise. No form designer, you have to worry about low level stuff like data binding, the default styling is too basic to be of use. It's a major step backwards, hence people being excited about Meteor (http://www.meteor.com/) - it's finally bringing back the RAD tools that we've been lacking for so long.