Stones are not ammos. that is why like bottles of biochem, they are limited. The literal reasoning there is that if snipers and gunslingers auto attack with a limited supply of arrow, that is easily depleted.
The problem here is that making the stones unlimited wouldn't really be to the benefit of the ninja, rather it would benefit INT type stalkers that copied the said type of skills. so, unless you can only make it unlimited for ninjas only(which is not possible in my view), -1 on this.