You're storing exactly the same data whatever you're storing it in. The point is that there's rather few use cases where your primary database is going to be mo

You're storing exactly the same data whatever you're storing it in. The point is that there's rather few use cases where your primary database is going to be more than a few terabytes, which is usually easy to handle with a single machine + some caching. I pointed to Stack Overflow as an example of a large site which still manages to keep its entire database on one machine.