With a decoupled front-end you can make applications which are much more flexible for dealing with change. Imagine swapping out the entire backend application, or completely re-writing the front-end - you can easily do either since the integration between the two is formalised and documented by the json contracts.
That also means a new front-end developer can clone down a Shunter application and start modifying templates with confidence it's not going to break anything without having to run the backend, services, mock db, etc. Great for working in big teams, or ones that aren't co-located.
It can also run a single unified front-end across multiple backend applications (for example if your rails application wants to share a common layout etc. with your Wordpress-driven blog section, or whatever ecommerce backend is driving your 'shop').