Sorry Garth, but you are wrong. With the update in CON damage not killing mobs, they buffed CK by adding acid damage to it. So you're right in the fact that it does no con damage, but it still deals acid damage and also add concealment. Also, the acid-damage is not poison based as I've seen it do damage to Epic Wiz King Guardians and to Harry.
As for FW, it does not stack when placed on top of each other, but they do still do damage if they are near each other. In general, I create a line of them and kite mobs through the whole line, works every time...