This works for small projects or sites (or if you want to ensure job security). But if you want to make something more complex, many of these JS frameworks can

This works for small projects or sites (or if you want to ensure job security). But if you want to make something more complex, many of these JS frameworks can really increase productivity. Ember.js, for example, has all kinds of nice buffering features, so you aren't making multiple un-needed ajax calls back to your server. You will also end up needing to re-invent the wheel if you make anything complex.