Just think of 5 thanes with + pierce stacking ae,don't think it works,give it to thanes as realm ability though,they need lovin !
Just a random thought.
For example: If you had 2 players with 10% pierce = 20% and attacking 1 target at the same time. Would the pierce % stack?
There used to be a bug with pets having 25% spell pierce, but that got fixed long ago for animists but stil works on necros's and theurgs
Fixed!
Nothing is a must, but the more stats, the better.. You can do just fine with no spell pierce, I can imagine