Would you have a support company or a contractor come on to your worksite if you knew they did not have insurance coverage? Would you do business with another company if you knew for certain they did not have liability coverage?
Of course you wouldn't!
Then why would you ever do business with a company, or why would you ever let someone work for you (on your property) without first verifying that they not only have insurance, but the right type and amount of coverage needed to cover any potential claim?
Every day you do business with third parties which are inadequately insured or are failing to protect your business as an Additional Insured on their coverage, you expose your business to unnecessary risk exposure.