[RobertMiller]

> They haven't used them because they are -- for all intents and purposes -- useless.

To your point, some numbers from wikipedia:

The LD50 of sarin is 39 micrograms per kilogram, so 0.0039 grams to kill a 100kg man half the time in theory.

Now consider the worst of the gas attacks in Syria was in Ghouta. Fatality estimates vary considerably, but the highest estimate claims 1729 fatalities. The attack is claimed to have been performed using at least 8 rockets, likely more, each with at least 50 liters of liquid sarin. That's a lower bound of 400 liters of liquid sarin, for an upper bound of 1729 kills. Liquid sarin's density is roughly the same as water, so that works out to more than 230 grams of sarin per person killed. And if you use the lower fatality estimates or the higher rocket count estimates, the numbers are even worse.

Conclusion? The delivery/dispersion of poison gas is incredibly inefficient. In practice you need tens of thousand times more nerve gas than the LD50 figures would have you naively believe.