Mongo with a strongly typed language also encourages someone to put some definition around the dataset. That someone is just the developer creating the PO*O (Plain Old [C#/Java, etc] object).

In C# for instance using the standard Mongo driver, you're dealing with strongly typed collections and the compiler is keeping you from mistakingly going outside of the "schema".

I don't know how good the other .Net drivers are for other Nosql databases.