I love using JavaScript up and down the stack, for me the biggest benefit is a lack of context switching. Instead of having to switch back and forth between different languages, you can stick with just one. It's really hard to program in
Java and be thinking about things in a Java paradigm and then switch to the front end code and now think about things in a javascript context. The two languages are so very different. Not to mention switching between SQL and JavaScript. In a traditional Ruby on Rails web stack we're talking about JavaScript, HTML, CSS, Ruby, ActiveRecord, Rails, SQL compared to a JavaScript stack of JavaScript, HTML, CSS. The all JavaScript stack is simpler.