Yes, you can just use Rails as an API and create a frontend SPA. But in my opinion, it's faster to write in the 'old way' for a MVP instead of creating 2 different applications (one frontend, one backend). For the future, I'm actually thinking about rewriting the frontend using SPA technologies.

Way faster. We are using a rails API and a React+Flux app as a front end. The whole setup time alone is a considerable pain. I feel that the JS tooling is still too fragmented and sometimes messy, but I'm probably biased by years of rails and iOS development. Writing the app takes more time since you need to write everything from the ground up (e.g.: no Devise views to just style and forget), but I really hope that in the future the investment will pay. I wrote about it a while ago (although now we are using the more terse Alt implementation): http://fancypixel.github.io/blog/2015/01/28/react-plus-flux-...

Completely agree - just setting up a workable environment for webpack is a disastrous exercise. Even things like cache-busting signatures, etc are a implement-it-yourself deal.