I'm no expert, but hopefully by abstracting out different services from your core app and making them available through some connection protocol (Thrift, a message queue, http, whatever) you can more easily scale up and scale out systems that become hotspots, and consumers of that service will never be any the wiser, since they only interact with it through the published API.

Looking at it another way, when I talk to the GitHub API, I have no idea what sort of database I'm ultimately talking to. Their interface abstracts away all of that complexity. Each request they receive from me might be handled by a different machine for all I know. It makes no difference.