This is something that we hear quite often in the mailing list, and I thought that I would spend the time to answer it in full. There tend to be two kinds of FK references in RDMBS, the essential ones and the one the DBA added just to make my life a living hell.
The first one include things that are essential for internal consistency within a single aggregate. For example, the OrderLines.OrderID FK reference to Orders.ID is quite important, since an order line without an order is meaningless. But what about the association between OrderLine.ProductID and Products.ID ?
An OrderLine can most certainly exists without the product, in fact, an OrderLine already copied into it all of the properties of the product that are important for the order. But because of the FK happy nature of most DBAs (and developers, for that matter), we have a FK reference between the two. The problem is that it actually is perfectly fine to remove a product that we are no longer selling from the store.
Yes, there are order lines for that product, but they have been completed ages ago. With a RDBMS and a FK, you cannot do that. So you resort to hacks like IsDeleted = false, which in practice gives you the exact same behavior as a deleted product, except that the FK is happy. Your application has a 50/50 change to work or not work with that.
With RavenDB, we make distinctions between internal consistency, which is maintained inside the same document, and external references, which can come and go as they please. You cannot have an order line in RavenDB without an order, because the order is where the order lineexists. But you can most certainly remove the product that an order line refers to, because that is outside the scope of the order, it is a separate aggregate.