I had a Shaman once who had a set of sacrificial knives made out of a variety of materials, including bone, stone, and obsidian. He also took advantage of the fact that Stone and Obsidian are much cheaper than metal in order to arm a bunch of mooks that the party acquired, not only on the cheap, but more importantly, in a fraction of the time it would have taken to make metal weapons for them.
This was in a situation where purchasing enough metal weapons or weapon-quality metal would have raised suspicions by the authorities we were working against, but the stone and obsidian could be bought without suspicion. We were also able to just source some of it as something that our minions could gather out in the wilderness.
I've also used them a couple of times on 1st level characters where I wanted to stretch out their starting wealth to make every single gold piece count.
They're good for Necromancers who practice minionmancy, too, making for weapons that are even more expendable than the undead wielding them. Even with Fabricate in the mix, it's significantly easier to find a supply of stone that you don't have to pay anything for than it is to find a bunch of iron ore or even bog iron.
Never have gotten to actually combine the fact that Stone weapons have rules AND the Stone Discus spell creates permanent stone, though, as much as it amuses me.
