I don't think this is quite right. Consider the hardest solo content available in the English and JP version: Accessory Quests, Guild Orders, Tower events. Of these, as far as I know only AQ gives you element-specific rewards. But AQ4 and AQ5 can be cleared with optimized same-element teams (caveat: Miracle tickets likely required). Guild Orders and Tower events give you rewards that can be used on any element. Therefore, if you main Light, having a Dark team is not necessarily more useful than, say, having a Fire team.End Game+
Based on DMM content, eventually, clearing contents at highest difficulty will like require elemental advantage. For example, completing Accessory Quest 5 on Light is still possible with a perfect Light Team, but your life is much easier using a decently picked Dark Team. Since the best way to farm high end Light gear is to use a Dark Team, and best way to farm high end Dark gear is to use a Light Team. In end game+, player is encouraged to have 2 teams.
An Elemental advantage build is a completely different investment strategy that relies on having elemental advantage in most or all content, not just some. The point is to invest equally in multiple elements and make all of them endgame-capable, based on the assumption that 6 good teams will be stronger than 1 excellent team for clearing high-difficulty content.