I agree. You always need a clean api to allow for multiple front-ends and to have a secure boundary layer between trusted and untrusted data. Even if the rendering happens on the server, you should always start with an api.

I work on a 400 kloc app developed with a client-side mvc + web services architecture. The nice thing I've learnt about having a proper services api is that slapping another front-end on top of it is straightforward. We've had a relatively easy time building mobile apps thanks to having the service api ready. And because the core app is built around the same services, they are less likely to break.