I think people only use it because it stores JSON and let's you query it arbitrarily on demand. It appears to be as good as CouchDB, but with on-demand queries, as good as Postgres, but with JSON (!). It appears to be quick and easy and the tool for the job.
Also, when people say NoSQL they actually mean MongoDB.
PostgreSQL has supported JSON for a good while now, I've even explicitly linked to it in my post. And no, MongoDB is certainly not 'as good' - many of the problems listed simply do not exist in PostgreSQL.
Also, when people say NoSQL they actually mean MongoDB.