The value of the warranty with Koetsu is that they will rebuild the cartridge in its entirety, keeping the original body. This may not make sense for a 4k dollar cartridge, but at a certain point, it is a fraction of the cost of the cartridge at retail, particularly in the U.S. Thus, when buying from overseas, it does have value to make sure the cartridge is from an authorized dealer if they are not fly by night and can send the cartridge into the factory. I’m not against third party retips but as everyone knows, the factory rebuild will have better value than a third party retip no matter how good. (I’ve read about a couple of retippers who have done Koetsu where the claim is "better" than factory, but I’m not sure that’s always the case). In any event, that’s the value of the warranty. If you have a stone body that is worth 13k plus in the U.S. market and can get it rebuilt for say, 4 grand, you have something of value.
I have two stone bodies, both U.S. authorized. I’ve had one rebuilt.