Thank you all for replies and explanations, it all makes perfect sense now to me
Especially the bit about some players can easily over use it in PBEM games.
As an example of this I am playing 2 games at the moment both in heavily wooded terrain. One player or other starts using Z fire without sighting the enemy, thereby giving away his positon and the other player responds knowing roughly where the enemy is now.
It is laughable but we have ended up in both these games with what I can only decribe as a Z feast of a game and it is like we are both firing at ghosts, each trying to suppress one another before we advance.
OK then I have seen some house rules on Z fire, but is there anyone here who uses them and swears by them so much so that they will improve situations like this.
Thank you