Spell penetration does nothing in PVE. At all. Ever. It is a good PVP stat and nothing more.
Boss level mobs have a 5% partial resistance to non-binary spells (i.e. those that can be partially resisted). This resistance can not be overcome with spell penetration or hit percentage. [note: This particular mechanic is still undergoing testing, some bosses appear to violate this rule].
Is it absolutely confirmed spell penetration does nothing in pve? don't some bosses have partial resists to fire or arcane and so forth?
Further, the two statements seem a little bit of a contradiction.
please discuss whether spell penetration is useful in PVE on level ?? bosses.
No known raid boss has penetrable resistance scores. Partial resists on non-binary spells are caused by the level difference between caster and raid boss (+3) and are the functional equivalent of glancing blows for physical DPS classes. These are not affected by spell penetration. The data-backed consensus is that spell penetration is a worthless statistic for PvE encounters.
I used to use an addon which the name of eludes me at the moment, that tracked estimated resistance of mobs. An average raid boss had something like 28 resist to each tree on average, as a base. I always felt spell penetration was worth having and using. In a situation where you have Curse of Elements (mage perspective) perhaps its not worth having.