You can build complex user flows with JavaScript on top of server rendered views. Ajax works great with server side rendering, see Pjax https://github.com/defunkt/jquery-pjax or Intercooler http://intercoolerjs.org/

The server is rendering JSON anyway, more or less just as expensive to serve HTML. Besides, the bottleneck is usually database access.

It's a false separation and forces you to expose far more than necessary. It's nice to have my full database available on every page render without needing to define explicit endpoints.

I also like having forms generated from database models.