Some designers and builders don't do things like this because of the "extra step" involved, and if you use proer wood it will be stiff enough anyway. But when you get to a certain point where you are willing to do the extra little things that add up to big gains or improvements, you end up with better flying airplanes and as a result you fly better. Another similar step is inlaying a strip of balsa like that in the central portion of a sheet surface to add stiffness and warp resistance. I have seen this on many British all sheet F/F sport models. Teaches you to be precise with your parts fit also. If the parts fit tight, minimum glue is used, and little or no weight build up and a stiffer, better part is the result. It all just depends on what you are willing to do.
Good luck and have fun,
Dan McEntee