The only reason I could see for doing this, is to provide the skipper something to hold onto when the windward hull dropps suddenly on the trapezze, thus preventing the skipper from flying off the back of the boat, and the time it takes to recover. The mainsheet offers no help in this situation because it just swivels around as you go flying off the stern. While at the same time with the crew holding the mainsheet, if the lee bow burries, the crew has support from going around the forestay (However this sounds dangerous here, if your on a broad reach or going to windward, and burry a hull, you would want to let OUT the mainsheet, not have the crew pull it in to prevent from flying forward.)

I would think the suggestion of having a line tied to the middle of the tramp for the skipper and crew to get support from while out on the trap a much better idea. As a the skipper you have much more control over the boat, being able to adjust the mainsheet to any steering you might have to do.

Just my opinnion,

Brent