Well, we already are using TS3. It has a very nice voice quality.
Mumble would only add the Directional talk, but ArmA 2 already got it. And we use it for short communications.
So, I see no advantages in using it
Mumble has been brought up in the past, please use the search function if you have not done so already to address the arguments that have been brought forth against it. Personally, I think mumble is indeed more advanced than TS3, but I am not the one who has to do the work to make it run.
Second, and I think that's the main point: In (probably) a few days, we will have ACRE which will in all likelihood remove the need for anything else but basic TS3 (no squad channels, no whispers lists etc.) and give full positional audio for direct speech and awesome control over radio channels.
Mumble has been suggested many times in the past and there are various threads which you can recall both sides of the argument. Remember that the switch to TS3 was not made by the Arma Admins, but by the Site Administration and the Owners/Operators of TG. Also remember, losing TS2 was necessary as TS2 became non functional with the forum update (new users could not register for TS2). Lastly TG does not own a Mumble server, therefore it would add onto the cost of site operation to get one, whereas we already have TS3 (with ACRE in final development). Let's not beat a dead horse here by reopening old wounds.
A lot of you need to remember that TS2 canceled a lot of it's license agreements and the one TG falls under was canceled. TG isn't considered as non-profit (I guess because of the SMs and any other means of transferring money to TG) and is why TG can't run a TS2 server which would be big enough for it's community. Besides, I believe even the non-profit licenses aren't being "sold" anymore.
Be advised that this is only what I can remember hearing from one of the admins a while back when the question was asked on TS, some or possibly all of this may be wrong. Please correct me if that is the case.
Damnit Blizzard, fix ZvT already >.<
In Soviet Russian, Arma admins are nice to you!