Nice discussion but.....
you are not considering the tow planes that have a winch system for tows.
There is a winch inside the fuselage that retracts the tow line after each tow.
At Williams they have a guillotine sytem to cut the tow line if needed.
For a dedicated tow plane this seems to be the most logical solution. Really saves wear on the tow rope and provides a very predictable way to "cut the cord" when needed.