This wasn't the standard or common practice that you seem to think it was - on the contrary, it was quite rare. In Classical times, I think it was only Crassus, Marc Antony, and Augustus or maybe Tiberius who actually did any decimatio. I'll look it up when I get home and give you a source or two - I definitely remember something from Suetonius.

So here's Suetonius on Augustus (from Lacus Cutius):

24 He made many changes and innovations in the army, besides reviving some usages of former times. He exacted the strictest discipline. It was with great reluctance that he allowed even his generals to visit their wives, and then only in the winter season. He sold a Roman knight and his property at public auction, because he had cut off the thumbs of two young sons, to make them unfit for military service; but when he saw that some tax-gatherers were intent upon buying him, he knocked him down to a freedman of his own, with the understanding that he should be banished to the country districts, but allowed to live in freedom. He dismissed the entire tenth legion in disgrace, because they were insubordinate, and others, too, that demanded their discharge in an insolent fashion, he disbanded without the rewards which would have been due for faithful service. If any cohorts gave way in battle, he decimated them, and fed the rest on barley. When centurions left their posts, he punished them with death, just as he did the rank and file; for faults of other kinds he imposed various ignominious penalties, such as ordering to stand all day long before the general's tent, sometimes in their tunics without their sword-belts, or again holding ten-foot poles or even a clod of earth.

Just to note, that although I don't think that there is any particular reason to doubt this account, Augustus was not a great military commander and may have wished to cultivate an image of himself as possessing stern antique virtues little seen in decadent modern Rome - and in any case Suetonius was born 50+ years after Augustus died in old age, so he's not a primary source by any means. There is a pretty common trope in Roman history of the virtuous commander bringing order to an unruly legion through unstinting discipline and then leading them to victory against daunting odds.