Last Stand = I am in danger of dying and I need to survive.
Rallying Cry = Damage on the raid is ramping up and the damage isn't as threatening to myself because I already have more effective health however my survival means nothing if all my healers and DPS are dead.
Firelands has a lot of instances where Rallying Cry is more useful than Last Stand. Beth'tilac on the second phase. Lord Rhyolith on the second phase. Shannox (normal) on the second phase. The list goes on. Basically if a fight involves a phase where there is heavy AoE damage that increases in magnitude the longer the phase goes on you will generally want to use Rallying Cry.
It's a matter of what is the current or growing threat. Raid damage or tank damage. If the tank is the only one to suvive, they won't last much longer. If the raid lives but the tank dies, the raid won't survive much longer either without a tank. It isn't a black and white just use this ability from now on.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.