From what I've heard, the next release will both add transformations and ingested syndromes, which could be combined to create all sorts of useful potions. For example, turning into a weaker creature that can fly to escape, turning into a more powerful combat form, exploiting the transformation mechanics to make a healing potion, ect. But if we want NPCs to ever be able to use this stuff, we need to be able to use raw tags to say what an item is used for. Some thing like [ESCAPE_POTION] or [COMBAT_POTION] maybe followed by some arbitrary number indicating how useful the item is. Obviously the system would have to be more complicated to specify how to use the item and to which creatures the item would be useful. The point is, we need some sort of system, because no way the AI could figure out on its own how to use some of the crazy things modders think up.
Also, about specifying the shapes of weapons, that seems like a needlessly impractical way to do it. For one, everything in DF is a weapon, so you'd have to go and specify the exact shape of every item in DF in great detail, including tools, furniture, rocks, logs, severed body parts, and plenty of other things. For two, such a system would be very cruel to anyone making a total conversion, since they'd be adding a lot of new weapons and have to specify of the shape of each of them, which would be much more work than the current system. Finally, the weapons modders are adding might not necessarily make any physical or logical sense, and a weapon shape system would greatly increase the amount of experimenting and general messing around that would be required to achieve the desired result.