The town of Falmouth mandates top swivels be used but some installers still do not use them.
"Each mooring must have one (1) top swivel; all swivels and shackles must be to the appropriate size diameter."
As the boat swings around the mooring, with wind and tide shifts, the chain twists and as a result, it shortens. This chain has shortened so much that the ball is being sucked under water. Short scope with minimal ability to retain caternary, especially in a storm, can literally lift the mushroom or granite block out of the bottom and the boat can begin to drag through the mooring field. It also makes shock loading of the hardware more likely with little to no caternary present.
While swivels are and can be a high wear item they can also easily be over-sized to compensate for wear. I use a 1" swivel on my own mooring and regularly get 4-5 years out of them. They have always out lasted my 3/4" top chain. They are also available in 1 1/4" and 1 1/2" for larger vessels. Hamilton Marine sells them in these sizes. Are they expensive in that size, sure, but a LOT less than the hull deductible for my vessel. My 1" swivel costs about $100.00 far less than totaling my boat..