The only time it makes sense to get a policy only covering personal liability is when you already have a package policy and need more liability coverage on top of that. (effectivecoverage.com)
You'll need to purchase this separately from your auto or homeowners policy, and it basically adds more liability coverage on top. (progressive.com)
Or you might want more liability coverage if you are often driving with children in the car. (simplethriftyliving.com)