You can't use the entire area at once, but you can use the entire area IN EVERY DIRECTION--up, down, left, right. So no matter which direction you're off, the additional area helps. Take a compass, stick the point either end of your 18" diameter line running through your IC pattern, extend it 2", draw a ring around your 18" pattern--turning it into a 22" diameter pattern. Thus compensating for a small aiming error no matter where you make it.
The other advantage you get is that if you use the same shot charge to produce both patterns, the cylinder pattern is significantly less dense. Less chance to blow up a woodcock at 13 yards. And remember, if the AVERAGE 1st barrel kill is 13 yards, some are going to be even closer than that. Sure, you can go with a lighter shot charge in your IC choke, which gives you a less dense pattern at closer range--but then you start to lose the advantage IC gives you over cylinder for longer shots, because you cease to have an effective pattern out at ranges like 30 yards, where IC should still be effective. But only if you don't go with really light loads to give you a less dense pattern for closer shots.
