Re: Falcon XX
"It clearly wouldn't be practical to unbolt every single nut after every single launch"
Getting an engine back after a launch, intact, allows the things to be torn down to see where the stresses are, etc etc - and by the time you finish that kind of inspection the parts can't be flown again.
Even the ones Elon's had back in bits have been useful.
Don't expect to see a used Merlin flying a commercial payload anytime soon (or if they do, it will be on a no-liability hitchhiking basis). The prime use of the first few flights he gets back will be redesigning components to ensure they can survive multiple cycles. That's the kind of data you can't get from pogoing out of a cow paddock.