I have some clients that require that I carry insurance. That mandate is is passed to them from their insurance policy. If I use a subcontractor to complete work for me, my insurance policy mandates that they have insurance.
It's all a big conspiracy by the insurance companies!
For me, I don't consider the policy that expensive since theses clients bring many, many times the cost of the policy to me in revenue.
In addition, it lends sense of professionalism to a new client when you disclose that you carry a x million dollar policy.