Proponents of shelf/footed mains will tout the "end plate" effect of the shelf reducing pressure loss around the "loose foot"... Personally I doubt that that effect is significant in the end.
The more uniform shape, easier outhaul loads and ease of bending and unbending the sail that comes with the loose foot (along with simplified reef line rigging, as per SD) come out in its favour for me.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"