The thing about liability is that in many cases it can't be waived. If I don't use proper safety practices, proper training, proper equipment, or do something similar that is less safe than would be proper, I'm liable regardless of what you signed. For instance, if I run a rock-climbing business and you sign a waiver and the rope breaks because I didn't do proper maintenance and inspections I am still liable. In other words, even if you sign a waiver that doesn't mean you can't expect me to take reasonable precautions and not be negligent.
On the other hand, if you decide to un-clip half way up the wall in spite of being warned and trained and all of the equipment being good and then you fall, I probably don't have liability even if you signed nothing.
Obviously, all of this is subject to everyone having good lawyers and anything can happen in court. The point of a liability waiver is more to discourage people from going to court because many people may accept that if they signed the waiver they can't hold the person liable.