MongoDB costs you nothing as well. And there are plenty of people who know MongoDB as well as 10gen who can provide official support if needed. It is quite a popular database you know.
And there are plenty of people who are switching away from PostgreSQL and other SQL databases to MongoDB. Again it is quite a popular database (hence the huge amounts of cash they seem to be easily raising).
Don't forget that a lot of people have realized that MongoDB is NOT a good fit for what they want to do. Then they have a resourcing problem - a big one.