Most personal auto insurance policies will cover a rental. The catch is that if in an accident any deductible you have will apply. You also risk of a rate increase with your insurance. The insurance you purchase has no deductible and will not be reported to your insurance company. However if bad enough to involve the police they will find out about it anyway. I have personal experience in this matter. I worked at a shop where a customer rented a brand new STS and totaled it. She took out the insurance and didn't have to pay a penny.
Bottom line is its up to you, most say its not necessary, some wouldnt consider going with out.