Elemental resistance penetration skills

Primeval Force gives 5% elemental penetration, and also Forces of Nature has 6% elemental penetration. If I'm already going for Primeval Force, how should I value also taking FoN if its nearby - is it better to stack penetration, or just a little bit goes a long way and adding more will give diminishing returns?

Does the value of getting FoN change if other cluster nodes are also considered (FoN cluster gives less bonus than PF).

Primeval Force gives 5% elemental penetration, and also Forces of Nature has 6% elemental penetration.

These values are pretty small. They're included more for flavor than for their game impact.

Both Primeval Force and Forces of Nature are primarily weapon/attack elemental damage nodes. Notice that the minor passives leading up to Forces of Nature both grant Weapon Elemental Damage, and the two paths leading to Primeval Force grant Weapon Elemental Damage and Elemental Damage with Attack Skills, respectively. You are expected to pick these notables for the elemental damage, not for the penetration. If your build does not benefit from the elemental damage, you are wasting skill points by pathing to them.

If you need elemental penetration, use an [Element] Penetration Support gem (which grants up to 37% penetration of its element when fully leveled), or a unique item which provides penetration of the appropriate element.