The guide for insurance is to insure the losses you cant afford to shoulder. If you ride a $500 bike and can put away $500 for emergency replacment, then you dont need to pay inurance companies to shoulder that burden. Use the money saved to buy a quality lock, a spare set of wheels, a backup bike and emergency taxi rides.
You cant afford the potential 3rd party damages so get covered for that. Some bike associations/clubs come with automatic 3rd party cover.