Attic Helmet too good to be true?

[Tarbicus]

So, to out do his rivals he outfitted some 2,000 gladiators in silver armor to enhance the spectacle. This suggests, to me, that ornateness was purely for decorative purposes, meant to woo the peasants.

There's a difference between all-silver armour, and decorated iron or bronze armour. A fifteen minute search of Lacus Curtius produces:

"O King," said Taxiles, "I could wish that some marvellous thing might fall to your good fortune; but when these men are merely on a march, they do not put on shining raiment, nor have they shields polished and their helmets uncovered, as now that they have stripped the leathern coverings from their armour. Nay, this splendour means that they are going to fight, and are now advancing upon their enemies."
Plutarch, The Parallel Lives
p559 The Life of Lucullus

"And most of all were his cavalry impatient for the battle, since they had a splendid array of shining armour, well-fed horses, and handsome persons, and were in high spirits too on account of their numbers, which were seven thousand to Caesar's one thousand."
Plutarch, The Parallel Lives
p545 The Life of Julius Caesar

"After this interlude, Totila himself, caparisoned in shining armour, adorned with gold and purple trimmings, rode out into the space between the armies, on a huge steed, hurling his spear in the air and catching again as he galloped, and performing other feats of horsemanship."
THE RECONQUEST OF ITALY (II)
(Part 3 of 4)
§ 9. Battle of Sena Gallica (A.D. 551)
p265

Non-Roman, but contemporary:
"The countless helmets and the shining armor of the warriors glowed like the rays of the sun."
A History of Armenia by Vahan M. Kurkjian
Chapter XX
Period of the Marzbans — Battle of Avarair

A 19th C writer's take on the subject:
"The soft or flexible parts of the heavy armour were made of cloth or leather. The metal principally used in their formation was that compound of copper and tin which we call bronze, or more p136properly bell-metal. [Aes.] Hence the names for this metal (χαλκός, aes) are often used to mean armour, and the light reflected from the arms of a warrior is called αὐγὴ χαλκείη by Homer, and lux aëna by Virgil (Aen. II.470). Instead of copper, iron afterwards came to be very extensively used in the manufacture of arms, although articles made of it are much more rarely discovered, because iron is by exposure to air and moisture exceedingly liable to corrosion and decay. Gold and silver, and tin unmixed with copper, were also used, more especially to enrich and adorn the armour."
Article by James Yates, M.A., F.R.S.,
on pp135‑136 of
William Smith, D.C.L., LL.D.:
A Dictionary of Greek and Roman Antiquities, John Murray, London, 1875.

Also:
"OʹCREA (κνημίς), a greave, a leggin. A pair of greaves (κνημῖδες) was one of the six articles of armour which formed the complete equipment of a Greek or Etruscan warrior [Arma], and likewise of a Roman soldier as fixed by Servius Tullius (Liv. I.43). They were made of bronze (Alcaeus, Frag. i ed. Matthiae), of brass (Hes. Scut. 122), of tin (Hom. Il. XVIII.612, XXI.592), or of silver and gold (Virg. Aen. VII.634, VIII.624, XI.488), with a lining probably of leather, felt, or cloth. Another method of fitting them to the leg so as not to hurt it, was by the interposition of that kind of sponge which was also used for the lining of helmets [Galea], and which Aristotle describes as being remarkable for thinness, density, and firmness. "

A description of the contemporary enemy against Sulla (note the effect of the rich armour on the Romans):
"The air could not contain the shouts and clamour of so many nations forming in array. At the same time also the pomp and ostentation of their costly equipment was not without its effect and use in exciting terror; indeed, the flashing of their armour, which was magnificently embellished with gold and silver,"
Plutarch, The Parallel Lives
p377 The Life of Sulla

Nor was Alexander (an admired hero of the Roman military upper classes) averse to putting on rich armour for battle:
"His helmet was of iron, but gleamed like polished silver, a work of Theophilus; and there was fitted to this a gorget, likewise of iron, set with precious stones. "
Plutarch, The Parallel Lives
p323 The Life of Alexander
(Part 2 of 7)

A Roman teaches barbarian allies how to be Roman:
"In consequence of these successes Sertorius was admired and loved by the Barbarians, and especially because by introducing Roman arms and formations and signals he did away with their frenzied and furious displays of courage, and converted their forces into an army, instead of a huge band of robbers. 2 Still further, he used gold and silver without stint for the decoration of their helmets and the ornamentation of their shields,"
Plutarch, The Parallel Lives
p37 The Life of Sertorius