This is like people saying why use jQuery or MooTools or prototype 7 or 8 years ago.
Within 5-10 years we'll be transitioning to a world where SPAs/native-like apps are the norm.
The way I see it at the moment is:
1. SPAs/native-like apps are really hard to do well still and the reserve of the coders with lots of skill/time to play
2. Javascript is still very hard to manage when writing anything non-trivial. Unfortunately, it's not getting better but actually becoming relatively worse when compared to the speed/innovation of other languages.
3. Browsers will carry on refusing to let us use any language but javascript[1] because 'standards'.
4. As javascript/the DOM are too unwieldy to write complex apps, we need the equivalent of jQuery for SPAs/native-like apps
5. Enter backbone, ember, angular, etc. They are our saviours against the mess of the w3c and javascript. They are presently in the early stages where jQuery/MooTools/prototype were in the mid-2000s
6. Anyone who doesn't get with the program, like this fella, will find themselves suddenly sorely lacking in a key skill almost overnight, as happened with jQuery
[1] Most shamefully Firefox, the ones who should have lead the charge 4 or 5 years ago to drop javascript as the one-true-way, will refuse to innovate past js. So much for 'free' or 'open'. Imagine if all Linux distributions suddenly conspired to only allow PHP 4 to be written. Or Windows only allowed you to write VB. It's such a bizarre situation we're in. The web is like the operating system of the internet but we're being denied the freedom to choose our preferred language.