When should you use MongoDB? I've asked that question a few times, and I haven't gotten a good answer I understand. I've read about a few of the NoSQL, document databases, and I think they do have a place in the world. There are domains of problems that they solve better than relational databases. I'm sure that streaming databases, graph databases, columnar databases, and more have places where we they shine, but I still believe that many, perhaps most, problems are best solved by relational databases.
I saw this piece about MongoDB being a poor choice by the Diaspora developers because they hadn't modeled their data well. It's a decent analysis of a real situation, and I think it's one that has probably been repeated many times by many developers that were intimidated, frustrated, or otherwise turned off by relational platforms.
There is a cost to using a relational platform, and it can require expensive developer time to map objects to relational structures. However the solution isn't to abandon the relational platform. The better solution is likely to train developers and hire a few DBAs that can help with the mapping and queries to solve problems.