A heater block is destroyed if one of the following happens
- Threads stripped
- Bent or otherwise deformed
- stripped grub screw
All of these can happen by handling the block with too much force when securing nozzles, thermosensors or heater cartridges.
Throats can be destroyed, especially e3D v6 throats with their neck down on the center can be simply turned and broken in two. Lined throats can be heated too much and the liner destroyed, which not always can be replaced, mandating a spare part. And you can strip the threads.
Another chance to damage the throat is by using very hard material nozzles - stainless steel comes to mind. Such a nozzle would not deform itself like brass when tightened against the throat and might lead to damage to the end of the throat if exchanged several times.
If you run several printers or change nozzles regularly for whatever reason, it is a very good idea to have at least a complete set of spare parts on hand to fix problems that might occur during work on the printer. I have a fully assembled spare hotend waiting for its day to shine in case my current one breaks...