Hybrid armor could be categorically lighter than chainmail (but less protective), and also less metal-intensive to produce, perhaps requiring only one bar for a chest piece. Hybrid armors could also be lighter than metal armor, which would be advantageous when outfitting one's scrawny little marksdwarves.
[ January 09, 2007: Message edited by: grendel ]
Say you tell your smith to reinforce some armour with bronze. He picks up some leather armour and does the work. Then the description would read "...blah blah it has been (quality) reinforced with bands of bronze". The defence would increase, but so would weight. Maybe this way we can create custom armours in terms of weight and material. Perhaps there would be a maximum number of reinforcements, maybe 2. Whatever stops it from being unbalanced.