Unstructured data is as bad as unstructured code. Typically, your data will become more complex over time, and structure helps a lot with ensuring that this is not a painful thing to do. Relational databases are well-studied and present a well-understood abstraction, with design patterns (normal forms) that significantly improve the maintainability of databases. Sure, relational DBs are not the only solution; I personally find Lisp s-expressions to be an easier to use Q&D store (of course, I am also a Lisp programmer, and most of my code does not have to deal with untrusted data), and lots of people are using YAML these days (and of course, there is XML).