Insurance does not help you the buyer. If the item is damaged it's the seller's responsibility, and Ebay will make him refund you, regardless of whether or not he has insurance. If he purchased insurance, he can take it up with the shipper, but you the buyer will never hear about it because you don't need to know about it. You get refunded regardless.
Personally I never buy insurance. Most forms of insurance are a waste of time and money, as the company will do everything in their power to avoid paying, and the paperwork is a waste of my time anyway. I've saved a lot of money by never buying insurance. Much more than I've lost in the 2-3 items I shipped that got damaged.
Sellers aren't even required to purchase signature delivery, unless the item is worth $750 or more.