Why herbal knowledge? Isn't it better to make good use of the spirits that never hit bottom with 1 lvl poison so that the spirit at least DOES something?
It entirely depends on how you choose to play the game, how much time you have, etc. If you log in once a day and spend all AP, herbalism is completely useless for you. If you log in a few times, then it may be useful as you recover hit points between battles.
If you ask me personally, I would always choose poison over herbalism, especially if fighting with a bow/crossbow. Its a quite powerful skill if you invest some points in it.