The waivers aren't worthless - they do help establish, or, if you will, minimize the duty of care, the breach of which is a necessary element to liability (assuming proximate cause and other requirements as against the tortfeasor/race organizer are met).
In the the rather conservative jurisdiction in which I live (Va), they do add considerable value, and the courts have upheld the waivers unless the plaintiff can point to a very specific act (or even harder, omission) of negligence on the part of the race organizer. One of the leading cases here holding the defendant liable had a triathlon starter on tape demonstrating before a novice race a "head" first approach to diving in a lake. The plaintiff unfortunately suffered paralysis as a result of such a dive. The plaintiff's lawyer, a friend of mine (and a fine triathlete himself) thought the case would have been exceedingly difficult but for the taped evidence.
I don't understand your point about the whole body of civil law being deemed pointless if waivers were uniformly upheld. The issue here relates to a voluntary recreational activity, with some risk involved. We aren't dealing with a city, for example, demanding a waiver before one can drive on their streets. Even if the waivers aren't uniformly enforceable (and they aren't - negligence can't entirely be waived and certainly not liabilities from intentional or tortiously wilful acts of misconduct), they do help mitigate the balance between risk and return for race organizers in a way that furthers more, not less races, with a cheaper cost, to boot.
In any event, puts an interesting spin on race banditry. Would bandits be willing to provide an indemnity (let's say one that is imposed by statute) to the race organizer against and all claims, with the added gloss that all obligations and liabilities under the indemnity are not dischargeable from bankruptcy, so that the liability would follow on around ad infinitum? If, as they claim, there is no "value" to what they are converting, then they ought to a priori accede to such an indemnity, right?