I forgot about your difibrillator example. If you actually use it it's not useless. If you don't use it you can say with hindsight that it was indeed useless (apart from giving you peace of mind maybe).

But it's a bad example because you don't own more or less defibrillator. Either you own one or you don't. And then it's not an ecoomic question. Because the answer to the question what is the right price for saving my life is always the same: All I have, all I can borrow and all I can steal.