Re: waivers and contracts -
I am NOT a lawyer, but my understanding has always been that neither will hold up if the plaintiff can prove negligence on your part.
So, yes, they have value and yes, you should use them. And yes, having them in place will help you in a situation where there is injury or loss of property, BUT they will not protect you if the accident, injury or loss of property can be attributed to negligence.
Also, there's a principle know as "the deep pocket." If there is an injury/loss of property, the insurance company will go after the indiviudal with the assets, regardless of most other considerations. The uninsured instructor? Unless they own signifigant assests, not likely to be a target. The insured propety owner, whether responsible for the accident or not? Yup, very likely to be a target, because there's the werewithal to pay a settlement.