Aaah, it is also used to build the OrderParameters. Now I'm unsure if we would want to remove this, or go the other direction and use it for all parameters in the documents.
Please review my [41c9a9b in my task/27180 branch] to see what removing it looks like. Maybe this can help us decide which way to go.
In fact, regarding OrderParameters, I think that these shouldn't use DetailsDocumentFields anyway, because parameter values and document fields are not necessarily the same thing. They happen to be the same in this case, and in most cases that makes sense, but there might be cases where they are not.