I have runners on my mizzen on an Allied Mistress ketch. I am pretty sure they were original factory equipment. The Mistress mizzen in only a little larger than typical yawl mizzens. I am not sure I would install them if I were to re-rig. They do add strength to the entire rig (there is a triatic connecting the two masts), but I do not fly a mizzen spinnaker or similar. They mostly serve to stumble over when going aft. Only used to ease the stay when off the wind to reduce chafe on the sail.
As a note, the mizzen is useless as an anchor sail. First it does not work, second it is much too large to leave up unless weather is extremely stable, third it would be noisy at night. However I do use the boom to raise/lower an outboard for the dinghy, but there are other solutions for that.