I would only reach for service-oriented architecture after the application is very mature and it is apparent which parts will benefit by moving to their own services.
I find rails in particular to be very easy to just "api all the things" right off the bat. Add in some nice authentication / authorization with something like devise and CanCan and you will be saved from the headache of bolting an api onto your app as an afterthought.
or the moment you have more than one application running. (which for many products is day #1)
rails is just a tool to build a front-end app. i have many of those on my project. some rails, some sinatra. if they want to access shared behavior or data, they include the appropriate gem which knows how to access a particular API.