Years ago, I made a very bad landing- quite hard - bounced - scared passengers....
I would have preferred the captain to "help" a little bit to avoid that, just as little as necessary not to make it so bad, and then give me a good explanation of optical illusions on narrow runways.
There are plenty of other occasions to practice and to learn...
One of my ex-bosses crashed during a NDB approach a few years before joining our company... airplane destroyed, no injuries thanksgod.
Do you consider that to be a useful experience? Wouldn't have it been more useful to have a good instructor really teaching (verbally and with actual example) how to fly a nonprecision approach?
There are many handling accidents which could have been avoided if the copilot had not been flying.
The typical captain's answer to the investigation people was: "Well, it was the copilot's leg, I figured he knew what he was doing..."
In the past, the difference in stature between Captain and copilot was enormous.
The Tenerife accident changed all that. Now our culture has shifted from one extreme to the other.
Many captains are afraid to intervene, when they should do so, to avoid harming the copilot's feelings...
There are many accidents to prove that. One of them is the Delta crash in Dallas Forth Worth, in which the captain still did not take over when it was clear they were in dire trouble....
and the list is long.