Decimation was a punishment for serious offences, like mutiny or desertion- not for performing badly in battle, or being cowardly.

With Romans, specifically, if cowardice was punished, there would be noone left. They were disciplined and had great quality equipment, but their morale was not the highest. For instance, before they met the Germans on the field, having heard of the military prowerss of their enemy, the Romans would spend their time crying and making their will, certain that they'd die in battle - Cesar has written about this.