I don't know what trees you are looking at but in many areas the colors of trees will vary greatly from species to species. So variation in the tree walls can be very useful. I would actually really like to have variation be an option on the kstree shader but we never got it. There has been work on custom shaders so maybe we can get it that way. Another note for tree walls is that it is best on the first wall to point the bottom normals down or at 90*. This will darken the bottom to better match the Y trees. Otherwise you will see the bright tree wall though the trees down by the trunks.