[ma2rten]

I have to agree with you that "developing two applications" is actually simpler, especially if your application matures and when you want start developing different kinds of clients (e.g. mobile).

However, my experience is that it still has some mayor drawbacks. Javascripts frameworks (testing and otherwise) are still less mature, then their server-side counterparts are. It is not always as obvious how things should be done. There are still lots of incompatibilities between browsers. And when an error happens you can not log it (or at least you have to do more effort).