I don't have my chuck key on a leash, but instead have a holder mounted to the drill press. I have seen others get distracted and turn it on while the key is still in the chuck. If it is loose it will fly out, but if it is on a tie, it might wrap someone up.

I work with equipment everyday, so I am not so much concerned about myself, but others that may be using it.
I do agree with the concept, but tend to worry about others, that might not be careful enough.
John