There are two problems with Foreign Weapons:
- Your dwarves haven't actually made them. This means that their quality is generally lower. I don't believe it's possible to make foreign weapons via a mood, like you can with foreign clothing. This means that you're going to have generally lower quality for your weapons, and that they will only be made of iron or bronze. Sure, an Iron Longsword is nice, but can it really top a ☼Steel Short Sword☼ ?
- Foreign Weapons are not optimized for your dwarves in terms of size. If you see a dwarf holding a foreign weapon with the "multigrasp" note, that means they are using both hands to hold the item, and so cannot be assigned a shield.
That said, there is one very tangible benefit to Foreign Weapons:
- Whips are crazy OP. Scourges, even more so. Dwarf Fortress doesn't model elasticity yet, which means whips wind up behaving like tiny, supersonic hammers. Quite honestly, its hard for a solid whip-strike to not chip bone, regardless of quality or material.
That said, their is nothing inherently "wrong" with foreign weapons. Feel free to reclaim as many as needed for your dwarves - put the large weapons in weapon traps, and give the militia the rest.
"Fun" Note: Large Daggers (a foreign weapon) pierce armor more easily than any other weapon. Part of the reason why Kobolds can be so deadly.