Again I havn't played too much yet, but so far it seems that the purpose of metal by and large is to provide a resource to sell to get $ (to get Crystals)

Perhaps its just been the amount of places to mine metal/crystals in my systems, but so far I've never had to worry about not having enough metal to do something - its always been money, and most often crystals.

This might change if I had to build more ships - as it is, my kill/loss ratio is pretty good because AI/Pirates arn't so good at assaults it seems once you get halfway decent force/defenses - however, ships are still pretty expensive in $/Crystals, so I don't know if that would change too much.