Sorry it this is retreading old ground, but this was brought up in the thread about separating couples at a wedding dinner, and I was surprised at the casualness with which some people suggested just swapping the place cards around. Surely there's far more to this then a guest could ever know about? The places might have been carefully chosen so that a shy/awkward person isn't on their tod, or that that Uncle Wandering Hands is kept safe, or any number of other things.
Leaving aside the couples together / apart etiquette, isn't it a bit rude to assume that you know better than the B&G?