Too many people just think "I need to store a lot of data on disk - I guess I need a database". They don't realize that databases COST you something too.
If its just about persistence - probably you can do better with plain old files. OTOH, if you really do have a lot of different ways of looking at your data, then a database is just the thing.
I'm actually moving an app from an oodb, which is mostly only good at persistence, to postgres because I have ever expanding reporting requirements.