@Frogbear: I don't want you to think that I'm just blowing off your comments. I don't disagree that the capability, properly employed, provides a devastating response. It can be very brutal on the receiving end.
What I would like you to consider is the other side of the equation:
1. The rule in and of itself has to be managed carefully. It is very easy to overuse the rule and a clever opponent will capitalize on that opportunity to the Tau's detriment. If the Tau is at an activation disadvantage, using the CoFire rule will magnify the deficit and severely hamper your (i.e. Tau) remaining options.
2. Using the CoFire rule early in a turn (piggybacking on #1), tends to turn the initiative over to your opponent in the latter stages of the turn. Because the Tau are not a "stand up and take it" kind of force, attrition in the latter stages tends to really hamper you in the next turn because Tau forces are rather fragile. So the rule does not provide equal benefits throughout the length of a turn.
3. The results can be a gamble. As pointed out by others, you "might" break the target too early, thus stranding or greatly reducing the overall effect of the CoFire. There is also the other side of the coin, the Tau player dedicates three formations to kill one of yours and in spite of the odds, it survives. Bummer for the Tau, been there, done that. It's just part of the game.
4. It's not as easy to set up as it might seem (try it sometime ÂÂ

), which is why you tend to only see it once or twice a game...unless everything is rolling the Tau's way, in which case it doesn't matter what the Tau do, you're just going to die. I've also experienced the converse as well ("No Shas'el, I do not know why I missed. Yes Shas'el, the Gue'la tank was only one meter away").
5. The Tau community is not that insular a group that we only play one army and it is our goal to make it superior to all others. Personally, I play Space Marines, Tau, IG, Elysians, and Necrons. I know others in the community play multiple armies as well.
Over the past year, the challenge has been to get the list to a point where it does fit in and play "nice" with all the other lists. We've had our share of bumps and detours, but it does seem to look like we are on the right path and that in the very near future we can finally put a wrap on this list.
So hang in there, please do express concerns, but let's also remember that it is in playtesting that we discover whether our concerns were founded or not. We don't claim to be perfect (I know I certainly am not), but we do strive to base our decisions on as much evidence as we can to ensure consistency in our decisions.
Cheers,