Why are you not supposed to put good knives in the dishwasher?

only if its a wood handle, soaks water over time warps the wood

if the knives get jostled around too much and bang into other silverware, it can dull the blade.

Also, if they are silver and fastened in a certain way, the dishwasher can separate the two halves. Or so they say...

I used to work in a restraunt and the chefs told me the hot water dulled the blade somehow. They would hand wash their knives by hand in cool water with soap then use an iodine water mixture rinse for sanitization. Their knives did not have wooden handles. The handles were some type of plastic that were filled with sand for balance. They were some type of German knives the they guarded with their lives. They paid hundreds of dollars for each one.

That's what I remember hearing - that it dulls the blade. But is there any truth in that?

The high heat in a dishwasher causes the blade to lose its temper and thus more readily dull through use. Once the temper is lost, the blade will no longer hold its edge.

Water, heat, jostling are not good for the microscopic edge on a really sharp knife. If you sharpen your knives frequently, then it’s not such a problem. Wooden handles though, yeah, totally screwed by the dishwasher.