andreas: I think the ability to have that could give information to the players.
Imagine an empty board with a white queen on a1 and a black rook on b2. The rook can obviously see itself and the queen can see the rook, but if you were to allow outside observers to see that the rook is visible to both players, then black could potentially become aware that her rook is under attack by a piece, when she normally wouldn't know this.
Obviously, there are other ways to deal with this (only show pieces which are mutually attacking each other, more complex yadda-yaddas, etc), but I really don't see how that would make watching an in-progress dark chess game any more exciting or worthwhile.