about the "weapon master" ascendancy note for merc ("Grants 20 Weapon Set Skill Points")

Weapon set skill points allow you to divert existing skill points on the skill tree to something else on weapon swap. "Grants 20 Weapon Set Skill Points" would allow to to divest even more points than normal.

An example would be to using a mace/shield combo on weapon set 1, with shield block nodes allocated; and weapon set 2 with a two-handed mace and increased damage nodes assigned in lieu of the former.

You won't get more skills points, but greater flexibility. Albeit, this my understanding of it at this time--I could be wrong.