It’s true that the lighter a model the better it will fly, the exception being a slope soarer in a good blow, and to this end it is always recommended that the tail end be built as light as possible. However, using very light balsa for the rudder and elevators can have unwanted consequences; I have had problems in the past and seen them on several occasions on other pilots models. Using “standard” bolt through control horns, the wood can easily be crushed on first assembly and / or work loose over time and piano wire elevator joiners can break through the balsa surface.
The answer is to use very thin ply, 0.4mm (for us oldies 1/64"), to spread the load. It is best to drill holes for the elevator joiner before fitting the ply doublers to both sides of the balsa. Cut the ply to shape and use these as templates to mark the balsa, make recesses for the ply, I use small files of this as it is easy to get a neat edge, then glue the ply in place. Once set, fill if necessary and sand over the entire area.
I tried to use a thumbnail for the photo but for some reason it wouldn't work; strange!