I do not have leading problems.
I shoot lead or most usually wheel weight bullets more than any other bullet.
Usually under 1000 fps.
But up to 1400 fps.
I shoot six guns, lever action rifles and semi automatics.
In 38 spl, 357 mag, 9mm, 9mm mak, 380 acp, 45 acp, 44 mag, 45 colt, 30-30, 45-70 to name some common calibers.
I have never had a leading problem to my memory.
I mean nothing that I did not clean up when I was done shooting.
I do clean guns after each shooting of 50 to 500 hundred rounds.
I have been asked about leading by shooting people I meet who seem to expect leading any time lead is used.
Some have stared down my barrels and comment how clean they were before I started to shoot.
I believe they are usually shoot jacketed an just expect leading all the time.
There are many applications where lead or wheel weight are just the ticket.
They are effective for fun, target and hunting, cheap to buy and cheap to make, relatively speaking.
My bullets cost me time only, now that I have the equipment.
My wheel weights are free.
Of course my time makes them expensive bullets in a sense.
I have had some brands to 22 lr to have wax (or something like wax) that when mixed with burnt powder, require me to clean six guns every 100 rounds or so. The six gun will just get gummed up.
I am not sure if this answers your question about when to use lead bullets.
But I gave it a try.
Earl