Gotta love the mechanics of Pierce in 1.09. A must for both bow in javelin users. Works real wonders with LF since each one calculates several times, the diminishing returns aren't as steep as they would appear. You might only be adding 1 or 2% to your odds to Pierce the first monster, but you have the same chance on each additional target, and Piercing 3 or 4x per LF is how you really cut down the big mobs.
For example, at lvl 10 you have a 71% chance to pierce the first target, and then a 71% to pierce the second one, if the first is successful. This works out to 71% of 71%, which is 50%, then 36% for the next one, etc. This cumulative effect is what makes lvl 20 so much better than lvl 10 for example. The first target is only 14% more likely to be Pierced, but the 4th target is more than double the odds.
Pierce is also the key to doing big damage with Guided Arrow, so you want as much as possible in it if you don't have a bow that has 100% Piercing as a property, which are not that easy to find anyway.