What can happen a lot with attack move is it will issue the command to planes that are currently re-arming. So instead of a compact swarm of magpies arriving, all of the re-arming planes are left with an attack move that's late and makes them stagger in slowly after the initial attack, usually getting shot down. Also, if you try to select all of your magpies and start issuing move commands, it will cancel the order to return to airpad. If you select all of your magpies and start issuing attack move and move commands intermittently to micro out of AA range, it wreks havoc on your airpad rearming. I imagine most people have all their bombers on a control group number, so there's no clean way to start microing your bombers without disrupting the re-arming happening back at base.
This is why we see air players often waiting for every single bomber to finish re-arming before issuing new attack commands.
This new widget gives an option to just stop issuing commands to the rearming planes by not having them selected.