While adamant arrows might be considered deadly enough, there are still those that would push it further. The Moon Garou hunters found that if they dipped their arrows in deadly snake venom, then their enemies would fall faster and with less danger to the hunting party.
Unsavoury bandit types found them useful as well, for far less practical a purpose. When they would raid caravans, they would strike the guards, drivers and occupants, with poisoned arrows, so that even if their victims could escape across the dunes, they would not live long enough to warn other caravans of their fate.