It would be advisable to have uninsured motorist coverage in the event that an uninsured driver is at fault in an accident hitting the motorcycle.  The accident may result in property damage to the motorcycle and injury to the motorcyclist.  If you don't have uninsured motorist coverage, your only recourse will be to sue the uninsured driver for negligence.  You might not recover any compensation because if someone is uninsured, he/she probably has few if any assets.