The lender coverage only covers you for the balance owed to the lender. That will of course decrease, as the value of your house increases over time. Owner coverage can help you cover for that. There is a really good explanation on About.com. See link below!
It is extremely rare for a buyer NOT to buy this. It is relatively inexpensive compared to the potential claim amount. For most home owners, buying a home is the single biggest investment they make in their lives. It is a one time fee and can help reduce costs if you should refi in the future. So, I would say YES, it is worth buying.