It really comes down to traction. On the road you still have plenty of tread but off road in wet and loose stuff you will notice the difference. Even on the road at some point you wear down past the sipes. I usually keep my rubber fresh. It gives me a chance to try other brands. Once my tires starts approaching to 1/2 tread I start thinking about selling the tires to at least put a dent in the cost of new ones. It's definitely more expensive than running the tires down to the wear indicators but it feed my rubber fetish. Used tires of popular models are always in demand. I would say the KOs fall in that category for now until the KO2s have been around for a while.
BTW, there was a recent thread on this subject. For details run a quick search.
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