[DoubleGlazing]

The problem there is what if you have two different apps connecting to the same DB. When the DB enforces referential integrity both apps can be certain the data in the DB is correct and they wont be able to mess with it. When the apps have to take on that role each app must implement (i.e. unnecessarily duplicate) the same integrity checking functionality. In a complex environment where you might have different apps using the same NoSQL data store that becomes very difficult to manage.