I grew up on rdbmses and think they are great. I've created numerous pieces of software backed by them. I have long preached the mantra that data is more important than the application. The data will almost always outlive the original application and become shared to others.

With that said, the above comes at a time/flexibility cost (not as bad as it used to be but still there) when building a product that isn't quite sure what it will be yet. In these cases a different data store can be beneficial since the app itself is key until traction is gained, if ever.