They need arrows that do one damage, and are solely for the application of poisons.
For instance, let's say I have a bandit I'd like to poison, but even an iron arrow might kill him in a sneak attack. I'd break out this arrow, apply my poison, fire, and watch as he goes berserk among his allies, runs for the hills, or dies slowly as a potent toxin creeps through his system.
Are there any arrows that are that weak?