So, if it is impossible to hold onto the main while being towed what would it be like if you had been tied on and the boat was still as insistent at dragging you through the water? Might there be occasions when this might drown you? Could you even be trapped under the boat under water by the tether?
A tether is one more bit of cord I won't be introducing as I have my life vest on at all times and a VHF and whistle in my smock when sailing distance. Hell, the tether getting caught might be the very reason you flip in the first place.