Skinlocks are highly optional. you don´t have to run any arch-class for pl´s (except maybe DL for melee classes)
You do still get a class PL bonus for running the archetypes that stacks 3 times. You are correct that it also counts towards the main class (e.g., Warlock and Skinlock, or Cleric and DA, etc.), so it is for some people optional.
Warlock:
Gives +3 MRR, this feat can be stacked up to three times.
Past Life: Warlock is a
Free Feat, which means it is permanently added to the character that unlocks it and you won't need to choose it, neither at new life nor at level up.
Skinlock:
You were an
Acolyte of the Skin in a past life. You occasionally find yourself throwing your hands into the air. Each time you acquire this feat you gain +1 MRR and +5 Fire Spell Power. This feat can be stacked up to three times.
Note: This class counts as a Warlock for the purposes of Completionist.
Past Life: Acolyte of the Skin is a
Free Feat, which means it is permanently added to the character that unlocks it and you won't need to choose it, neither at new life nor at level up.
If I wanted to get both of these PL benefits stacked three times each, I actually
did need to run both the standard Warlock and Skinlock.