I think the thing is, for that kind of bonus to attack, it has to be a stunt, or a maneuver, or something along those lines. It's a much more active thing than full defense, since full defense is, basically, a compensation for skipping your turn.
See also the Berserker stunt (+1 to attack roll and to damage, -1 to defense and you can't do anything but attack til your target's dead). I also came up with a gun-centric stunt along similar lines (Shooting Position: When in such a position like kneeling or prone, +2 to Guns attack rolls, but -2 to all defense rolls).