A long time ago I remember reading somewhere that very few knights wore full armour. It asked 'how many people do you know own a Lear jet? That's the equivilent of having a full suit of armour like you see everyone wearing in the movies.

After battles were over the survivors would scavenge the battlefield for whatever armour could be stripped from the corpses (as well as boots, clothing, coins in their purses, anything really). So they would henceforth wear whatever they scavenged chestpiece, greaves, etc..

The few that had full impressive armour would be marked as rich. These men would never be killed but captured and their families would be forced to pay a ransom for their safe return.