Without the halyard locked, the cleat can be pulled right off the mast. (Pulls the rivets right through the plastic cleat.)

About 10 years ago, I saw a guy who was using the tail of the halyard as a tow line. He didn't tie it off on anything, so all the load was on the cleat. It didn't take too long before the cleat pulled off the mast - even at a slow towing speed. Once the cleat pulled off the mast, he was being towed by the top of his mast. You can tell this story did not have a happy ending.