This is a feature that simply hasn't been relevant in any of my BNW games so far. I find the requisite spy-as-diplomat in the capital too restrictive to be any use until the late game; at that point I'm often sufficiently dominant enough to buy enough votes to 'beat' all remaning civs anyway. It makes sense for it to be implemented that way but as it is I simply don't use it, which seems a shame. Perhaps being allocated one diplomat for every two spies might help? Or maybe be able to discuss WC trades with a DoF?