Hi Giles Wembley-Hogg, as I said in the beginning it would be better to select TA only if we were sure everybody else has got a functioning TCAS.
Unfortunately, for Murphy's law, the day you are going to have a close encounter during an emergency descent, the other guy will have a transponder with NO TCAS
Thus my choice is for always leaving this device fully ON.
If a momentary leveloff will prevent a collision during an emergency descent, so be it.
That's the price to pay, and not a big one I think.
But at least you'll be sure not to hit anybody.
Regarding the "not doing anything", I repeat AT WORST don't do anything.
Don't climb if you have a Descent RA, and don't descent if you have a Climb RA.
Keeping the same path is not considered a maneuver OPPOSITE to the RA, I think.
It's bad, but not catastrophic.
This possibility is accounted for by the TCAS, which will instruct the other guy to increase his maneuver.
At Uberlingen, the system failed because the Russian maneuvered opposite to the RA.
Had he maintained level flight (yes, in the red arc), there would have been no collision.
I would definitely follow the RA even in an emergency descent.
Cheers, LEM