Well, there is a significant difference between wind, earth, fire, and water versus light and dark. Light and dark have the advantage of never being in elemental disadvantage (i.e. take more damage from and deal less damage to an element you're weak to) while still being strong against another element (Light <> Dark). The main elements will be disadvantaged against another main element (Fire > Wind > Earth > Water > Fire), but strong to another as well. Also, at the risk of over-generalization, light and dark generally perform better than main elements with similar grids, and thus at end game (it just takes longer for light and dark players to reach the level of development). So you have two options. Since you rarely only develop one grid, you can always start out with a main element, get strong fast, and develop other grids. Or you can consider developing a light or dark grid and sticking with it for a long time, which is doable precisely because you will never suffer elemental weakness, and in theory you can just stick with it forever because light and dark will never be a bad choice in any given setting.