the romans didn't use lead because it was an incredibly sharp material and held an edge well, they used lead specifically BECAUSE it was a soft metal, and would break off after one use, so they couldn't be picked up and thrown back at the legionnaire who threw it =P
Actually no. They used it so the tip (which was 12+ inches) would bend after penetrating a shield, making the shield heavy and unweildy, slowing their enemy down... that it was useless to be thrown back was a nice secondary effect.
My point wasn't that it's ginsu sharp, my point was that historically lead was used in weaponry. I'd have mentioned lead shot for slings and such, but DF doesn't have those weapons.
What about lead boulders for catapults?
Actually actually, lead was used to add weight to projectiles (javelins and darts) so they would have more impact, and penetrate deeper than a lighter weapon. The bending was caused by the shape of the bronze or iron head, which was a large blade with a very thin piece of metal connecting it to the shaft. When the point was stuck in something, the weight of the shaft would cause the thin spot to bend. Lead blades or javelin tips would be about as useful as soft butter.
Lead shot, for slings, cannons, and guns, is about putting the most mass in the smallest place for the least money. Just like in DF, platinum would be a more effective material, but the cost is prohibitive. Also, slings and (early) cannons often used stones because they were cheaper and more readily available than lead.
You can fire galena boulders from a catapult, but pure lead won't count as a boulder, so your siege operators won't use it.
JMZ