It's not just Pure Skill that is worthless, Pierce is worthless too
With the free mastery reset, I decided to do some testing on Pierce to see how it is currently performing. I tested using 5* Storm against RoL WS with full mutant support synergies so I would crit frequently. I tried Pierce at all 3 levels with a few different mastery setups, and pretty much without fail, the following is the bottom line:
Each point in Pierce increases your damage on critical hits by ~1 to 1.1%. So, if you crit for 3000, each point in Pierce is adding just 30 damage.
I don't know about the rest of you, but this seems like a waste of mastery points to me. Much like Pure Skill, the juice is not worth the squeeze.