I wrote "I don't know if there's a use case. I have never had one come up in my own projects." I don't work for Facebook or Google, perhaps they have a use case. I guess I could assume there's a use case because the tools exist and people use them, but in my experience I have not had a need for Mongo or any NoSQL database.

I'm not dismissing the tools out of hand -- I haven't used them except to play around with tutorials. I am pretty sure a lot of people start out with Mongo or whatever without having a use case defined and without having good reasons to eliminate the tried-and-true RDBMS solutions. "Use something cool and new" is not a use case. "I've read on slashdot that SQL databases don't scale" is not a use case.