Aside from crippling you immediately, vomiting lowers your morale, dehydrates you, starves you (since you lose all the fuel in your stomach), and just makes you feel lousy in general. If we managed to pipe a puke-gas into an enemy hideout, then seal them in, the stench would be unbearable. Nobody could keep anything down, and it would be just obscenely cruel.

And effective. I mean, U.S. troops could always strike from upwind.