Stormy Star of Elune

Penetration being "totally useless" in PvE is an overstatement. It's useless enough that you can safely ignore its existance, but there are some Felhunters in Hyjal trash with very high resists, and the second-last boss in Black Temple has a resist aura that's up half the time. Penetration would do something against these mobs. Whether it's actually worth using is another matter, especially since the situations where it has any effect at all in PvE are so extremely rare. So perhaps we can call it "effectively useless". :)

Anything would be better. Penetration is totally useless in PvE.

Spell penetration means that when you shoot a spell at that person, it lowers their resistance to everything.

So if the target were to have 50 fire resistance, your spell will act as if the target only had 40 fire resistance.