Javascript is probably THE language of the web. You can create awesome user experience on top of your – already awesome design – but you can also drive users to hate you and your site.
As a programming language it has to run to keep the things doing, but it is also very temperementfull when it breaks.
Javascript errors in production environments are the most bad thing that can happen to your customers, his users and you as a developer. You have to act immediately to find, understand and fix the error. This is my personal horror scenario as I dislike working under pressure and probably creating new errors while fixing that one.
Wouldn’t it be wonderfull if your javascript fails safe without annoying the user?