Misunderstood intention. What I meant by relevant was simply having a good enough grid to continue tackling content without any trouble. And mind you, future content simply favours the element advantage more simply by nature of there is only so much damage you can deal/ take. There really isn't a good phrasing for this. Maybe "don't want to be frustrated with difficulty" works.
Think of it this way. Focusing on a single element all the time limits your options to whatever that element gives you, as opposed to going for multiple when you have the resources. Keyword being you have the resources. Because as I link to the 2nd paragraph of the previous statement, there's only so much you can do for your grid for one element before you get capped. What to do next? 1. Whale. 2. Get another element setup.
The key point is this: nobody can dictate how to play the game. But when giving advice... I'd prefer giving the most optimal one. It's too difficult to account for a singular playstyle at any one time, not especially if the player doesn't plan to commit.
Edit: An extra titbit. All future tower content missions have 1/3 of their medal weight geared towards elemental advantage. Uhh... weigh this as you will.