Seems to me if you start writing your app with a service oriented architecture in mind then you have the flexibility to make your calls to services either in memory function calls or HTTP requests/RPCs.

One example case: you write a monolithic Rails app that is structured around services but all services are in memory service objects. In production you find that your search service is both doing far more work than your other services & getting queried more often. So you refactor your application so that a call to the search service is no longer an in memory function call but instead an RPC to a group of servers that only handle the search service and have been rewritten in Java/Go/C++ to be blazing fast. Since you wrote your app with services in mind, you probably don't change the API at all, it just becomes a wrapper for an RPC rather than a class in your monolithic app.

This way you don't automatically bulk up on unnecessary, expensive HTTP requests but you maintain the flexibility of optimizing modules of your app for performance.