First of all, note that Leadbetter flares both feet open. Many golf instructors recommend a 10-30 degree flaring-out of both feet, because it allows the hips to rotate more easily during the swing. Flaring the foot open allows the knee to move more easily in response to rotatory movements, so that the knee can re-position itself above the foot. This is especially relevant with respect to the left foot during the downwsing, when the body weight is driven powerfully across to the left side by an initiating lower body shift-rotation. Flaring the left foot open allows the left knee to first move laterally and then straighten, thus supporting the body weight directly over the left foot as the lower body pivots around an axis centered in the region of the left armpit.
Regarding the driver, Leadbetter recommends that the inner distance between the feet should be equal to the shoulder width.
Regarding mid-irons, Leadbetter recommends that the outer distance between the feet should roughly be equal to shoulder width.
Regarding short irons, Leadbetter recommends that the outer distance between the feet be less than shoulder width, but not significantly less than hip width.
Some golfers, who lack flexibility, may personally find Leadbetter's driver stance width recommendation to be too wide, because i) it prevents their hips from rotating fully during the downswing, and because ii) it therefore prevents easy/complete weight transfer to the left side during the downswing and followthrough.
Another method of finding the correct stance width, that takes body flexibility into account, comes from the book "Total Golf" .
Stance width variations - from reference number 
The "Total Golf" authors recommend adopting a stance width, and then turning the body 90 degrees so that the chest and hips fully face the target. The correct stance width is the stance width that allows the knees to get together and be in-line and the hips to fully rotate 90 degrees (number 2 in the above sequence of three stance width positions). Stance width variation number 1 is too narrow and allows the hips to over-rotate, while stance width variation number 3 is too wide, preventing complete hip rotation.
I think that each individual golfer should experiment with these two stance width approaches to find the best balance between rock-solid stability and easy hip flow. A stance width that favors stability over easy hip flow may be preferable when using a driver, while the opposite bias may be preferable when using a long/mid iron. Most PGA tour players prefer to keep their lower body quiet when using a short iron, and they try to avoid excessive lower body movement during their short iron swings.