It is worthless because as far as I am aware Blizzard no longer authenticates their presence. Sure they can be attached to your BattleNet account but Warcraft III still requires just a RoC CD key to function.
Since TFT CD keys are not authenticated, a generated or place holder one could be used for legacy purposes of installing and running TFT. The RoC key still must be genuine since that is still authenticated and used to play Warcraft III.