Wouldn't that be like a spiked club? Piercing dmg that is
No. A machauital functions more closley like a saw. Altough I do see your point, they called it the Horse Hating Sword, you could saw a horses head off with the weapon supposdly. I believe there is a tv show episode (Deadliest moron or whatever?) basically they show you cutting through that level of thickess. I see why you would say that it would have piercing though. I mean it sort of pierces yeah but the bulk of the damage comes from sawing. It's more of a aztec weapon too... sory I'd explain better but I realy can't type now. it's like pullin teeth on this computer.
Yes, and the issue with Stone weaponry is that people will not use it, the reason Abber Nomads dont use metal is similiar to that of the AThasians, its hard to come by metal gear.
Why would nobody use it?
However, maybe druids would actually start listening to their vows. 
This is actually a good reason why too, to be honest. I've seen a lot of druids using a lot of steel and metal... Chitin kind of allows them to... I guess "Loop hole" a druids way into plate armor. A stone craft would kind of help loop hole their way into good weaponry so to speak. Stone Weapons were very weak of course, but Obsidian still had it's use. Very sharp too. I mean, cut a horses head clean off with an obsidian saw...
Druids, primitives, and maybe even feral caliban can kind of benefit from an idea
EDIT:
I'd type more and explain thins better but this keyboard I have is curently garbo. None of these Key boards are good no matter where I sit.
