Think of the implications for ammo -- dropped arrows could be melted in one job instead of 30!
I don't think it's going to be that simple for ammo . . . does your masterwork giant kea bone bolt stack with your exceptional giant kea bone bolt? I'd be happier, I think, if all ammo just disintegrated on firing.
However it works, though, it's going to be awesome. Hooray for stacking and hauling!
There are some tricks that can be performed if you segregate out the data cleverly, and you can do something like how the stocks screen works - the game will tell you that there are 1000 or so stones in the play area by simply grouping everything with a "stone" token, and then break them down into individual materials.
You might be able to tell dwarves to pick up 50 "metal bolts", and the dwarves will pick up bronze and iron and steel alike, and with varying item qualities, and display them as a single stack in a "less detailed mode", then tell you that if you look closely, it breaks down into 15 =iron bolts= and 10 steel bolts and 25 *bronze bolts* or something.
Likewise, almost all bone uses the same material definitions, so you can just declare all "bone" material stackable with other bones, and just keep some separate tags saying where that bone came from. (Although I still wonder how a dwarf can just look at a bone bolt and tell it came from a water buffalo and not a cow.)
(Although this might cause problems if/when we get to the point where we start having alchemical/magical ammo like acidic or freezing or exploding or healing shiv bolts that get mixed in with "normal" ammo unless there are clear definitions for what gets stacked with what.)
Of course, there already are
nauseatingly in-depth threads on these subjects if you want to go further into the matter outside this thread.