CapeFear, per your last sentence, this setup doesn’t increase rack capacity, but it does effectively prevent unwanted movement which will tend to lead to rack/boat/fastener failure over time.
My initial choice of V-bars was to accommodate a long boat with no attachment points at bow and stern. Once in the cradles, I attach tie-downs to the ends of the carriers, and loop them around the boat.
On regular boats with attachment points, the 8 or 9 ft separation of the cradles still effectively restrains bow and stern from moving sideways on the car. I use a bow tie down attached to the boats, but only one is needed. Its function is to prevent aerodynamic uplift at the bow, since lateral movement doesn’t happen. The whole thing is extremely stable, especially with two V-bars which create a frame when attached to the cross bars.
It is true that the location of the cradles is adjustable, so they can be located near bulkheads and put less stress on the hull. Also, the long central bar flexes vertically and acts as a shock absorber, allowing the boat to move vertically a bit, reducing jolting to the hull (in the manner of a leaf spring). This is easy to see through the sunroof while driving on a bumpy road and is a significant benefit in my opinion.