The recommendation by the auto insurance industry is to buy as much insurance coverage as you can afford.
If some one is injured in an accident and you are found legally responsible for medical bills that are more than your insurance coverages, you will have to pay the difference out of your own pocket. These medical expenses can pile up quite quickly and if they exceed your coverages your personal assets may be put at risk.
The Insurance Information Institute (III) recommends that you have $100,000 of bodily injury protection per person and $300,000 per accident. If your net worth is more than $300,000 they suggest you consider buying additional liability insurance. You may also consider purchasing an umbrella or excess liability policy.
While most of the insurance industry, including the III, suggest 100/300 as your BI limits there may be others in the auto insurance industry that do request higher limits such as 100/500.
For instance we know that many volunteer services suggest that those carrying passengers get the higher limits of 100/500 to have that extra protection in case they are in an accident.
With the cost of medical expenses if one's financial situation can afford higher limits of 100/500 then they should do so in order to protect their assets.