The evolution of the nut tool, like many things designed by humans, does not exactly inspire confidence in our future prospects. It took more than 20 years to figure out it would be nice to have a carabiner-like clip built in, and another few before some genius figured out that welding a little round bit on the end of the handle would allow the user to strike the tool without slicing open their palm.
So far as key chain biners and telephone cords are concerned, I try to avoid mixing stuff into my climbing gear which won't reliably hold my body weight.
For me it's the Metolius one with a clip and palm protector riding on my harness with no cord. Look out below!