Here is a video of whats happening when you shift gears. The whole idea of a synchro assembly is to match the speed of the gear shaft with the teeth on the gear itself. So when you "miss a gear", what actually is happening is the shaft and the gears are spinning at different speeds and dont mesh when they should. That causes the grinding sound and since the blocker rings are generally made of brass, they will wear out the fastest. The teeth wear out as well as the cone portion of the blocker ring. I have not opened up a ZF, but many Borg Warners when I was wrenching for a living. We used to rebuild them in house when I worked for Anheuser Busch many years back. The principle is the same.
https://www.youtube.com/watch?v=0Bqs-oHBBQk