Your main argument seems to be "What are Spell Penetration feats competing with?". Indeed, this is the best way to think about it.
My main argument is that your question is wrong. Or rather incomplete. Do spellcasters need spell penetration? The answer is "insufficient data for a meaningful answer".
The list of enemies against which Spell Penetration works is useful, but to get a real answer we must start elsewhere. First, you need to decide: What does your character want to do? If he DOESN'T want to be casting spells affected by Spell Resistance, then the answer is easy. He doesn't need Penetration feats, end of discussion. But if he DOES want to cast such spells, then we need to start talking about opportunity cost. What does your character gain by taking those feats? The benefit that he can use those spells when fighting against those enemies (and it is not an irrelevant benefit at all. In the Siroket fight, for example, you need a way to deal with the Erinyes fast, before they decimate your party from afar). What does he lose? Whatever benefit he would get by picking some other feat over Penetration.
Now, you may still think the tradeoff is worth it. But you need to be aware of the cost of your choices to make informed decisions about them. Examining the benefit of a single choice in a vacuum is only a small piece of the puzzle.
PS. Shake it Off is freaking awesome, and I unite my voice with everyone who mentioned it here.
You guys seem to be clumping your party way more than I am. I can see it for melee, they're next to each other anyway, but not for ranged guys. How do you deal with enemies in your backline engaging all casters at once, or AOE attacks like the Spawn's?
This is all very well but Wild Hunt are very capable of Dispelling so all those protections can easily get dropped
I can honestly say that I've never had a problem with buffs getting dispelled in KM at any point before the Lantern King.
You guys who don't use Blindfight, what buffs are you using that provide the same effect?
Freedom of Movement protects against Paralyzation. Holy Aura protects against Mind-Affecting effects. And Echolocation lets you ignore concealment, even if it doesn't protect against Gazes (thanks Desiderius). Though it really should, for the same reason Blindfight does: They emulate being unaffected by closing your eyes, as you can do in tabletop.
You should easily be able to have those buffs up whenever you need them by the endgame. What are you using those spells slots for? (again, opportunity cost) And why do you not have consumables of them?