I don't remember seeing that predicated as a low-level I/O savings — it was always things about being easy to cluster or avoiding the performance overheads of ACID. The pitch was usually “it's easier and you don't need a skilled DBA”.
What I heard was "You can practically scale indefinitely". Which I guess is more true of NoSQL. As is I/O savings. But it's not something I've ever heard it advertised for.