You want Arms Lore for the additional resists you get when crafting any type of armor. It *used* to be useful for the 40 DI on weapons, but since DI is probably the last thing you really want to modify on weapons with imbuing, it's really useless for imbued weapons.
Arms Lore doesn't do anything else for weapons, doesn't increase the % of exceptional or anything. When crafting with runics it doesn't add % to modifications or anything else.
I guess to answer Gim's original question, Arms Lore plays no part in crafting weapons for imbuing. (with the disclaimer that if you imbue the weapon you'll reduce the DI to 1 anyways).
The point of reducing DI to 1 as a first imbue is so the other imbues that use Runics will be much easier and your success percentage goes way up because the total points on the weapon is much lower.