The ROTJ shot has a center beam.
It would probably make more sense if just a smaller beam from the emitter was fired at the fleet.

But probably not as dramatic as it plays out in the movie. Oh crap! It’s operational!

As it stands, it’s like using a tactical nuke on a battleship. If the DS beam could cause an blast in empty space, a shot right into the center of the Rebel fleet would wipe them out, or severely cripple the ships furthest away, but the movie would also be over. 😉