You're getting it. Change your goal from clearing content (milestone-focused) to strengthening the team you use most (rewards-focused), from being as strong as possible to being as EFFICIENT as possible, and suddenly an elemental advantage build seems like a lot of extra work for only marginal gain. Notice that much of the difficult content in the game (e.g., high-difficulty Disasters, Guild Orders, Tower) is either multiplayer or gives rewards that aren't locked to an element (but ultimately, have to be spent on items whose benefits are locked to a single element). The end result of this is that those who rely on elemental advantage are able to farm faster, but this advantage in efficiency is completely offset by having to spread those rewards 6 ways.