Leather Armor? (NO HOLDS BARRED!!)

[Tarbicus]

Aside from in siege warfare and fighting other Romans, how much did missiles factor in their use against a legion in battle, for which the scutum is the main defence, as a proportion, versus close combat against handheld weapons?

And this is only something that occurred to me: One advantage of leather over metal armour would be agility. An officer in leather would be better able to command directly by running up and down the line giving orders over a long period of time, unlike the metal clad rankers whose main purpose was to hold their ground and push forward, with spurts of speed under certain circumstances. Just as velites wore only skins because they needed to be agile and (apparently) be easily identifiable, but a tribune would need more protection as he would get in the fray at times with a more formally organised unit.