This is a very powerful passive skill since it affects any and all missile weapons you employ a chance to damage, pass through, and then damage another creature. When using a Plague Javelin or enchanted Arrow, this can be especially devastating. Also, consider the damage potential if you are using the Strafe skill.
Some recommend maximizing this skill at 20, however, the benefit of additional points drops off significantly. You can decide how many points are enough for you. If you cannot think of a better skill or spell to spend a point into, spend it on Pierce.
Lead monsters into a line so you can take full advantage of Pierce.