That could make printing a packing list very slow because the BOM components are not on the packing list so the rollup would have to happen for every line on every order all the way down through the BOM.  And in a machine shop environment, the weight of a forging is generally a lot more than the final weight of the machined part that comes from the forging so simply adding up component weights wouldn't work anyhow.

..yes I agree that there was a filter it would work faster. Also if the BOM for a product was calculated during a rollup when the BOM is originally created or existing BOMS could be rolledup such as Standard cost are.

Depending on how you want to use the weight, you could create a non-inventory item called Finished weight .  Then you could add that to the BOM.  It could be attached to the sequence # such as the packaging operation on a traveler if desired.  Unfortunately, this gives you the total weight if you get all the parts, but it would be somewhat of a comparison.
 
The other thing you could do is to put the finished weight per piece in routings under notes or specifications.  It would pull through onto P.O.'s and also show up on travelers.