Sailing downwind the main would simply go into a deep pocket like a jib.. the main would readily collapse and fill like a headsail sailing too deep. Also you'd have no way of maintaining leech tension ( no vang)
We've sailed on a Farrier F25C trimaran that was rigged with no boom, but they virtually never sail with the wind aft of the beam regardless of heading.
1984 Fast/Nicholson 345 "FastForward"
".. 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)