In Missouri, you are not required to provide worker's comp below a certain number of employees. I think it is three.
That doesn't change the obligation of anyone hiring a small businessman as a subcontractor. If he gets injured on another contractors job, he can sue the primary contractor and get paid under their WC coverage. Because of this, ALL companies get audited by their WC insurance company for their wages paid (to see if they are cheating) and also for matching coverage provided by any subcontractors. If there is no cert of insurance provided by the sub to the primary, the primary contractor pays for WC for the sub, even if the law does not require him to have it.
An interesting side note: An uninsured tree climber, working for himself only, is NOT required by state law to buy worker's comp coverage on himself. If he gets hurt on the job, he can sue the property owner for damages, medical bills, loss of income, etc for any injury occurring on their property. He might even be able to sue them for injuries sustained OFF their property, if he could prove a link between their work and his injury. (say...unloading their logs at the dumpsite.)
Worker's compensation insurance does not insure the injured workers against loss. It protects the employer from losses they would be liable for if an employee is injured "during the scope of their duties".