The flash is gameplay wise purely for visuals. There is no positive gain from the flash other than just making it visualy look strong. Just like a car can have a very powerful sound, but still be extremely slow in a game.
These things are only there for immersion reasons. Yet suffer from the part that they actualy do not provide any immersion for a skilled player. A skilled player just sees which summon it is and already remembers how strong it is. Other than that they do not need to know anything. Hence just an empty summon animation will do for them.
As the flash has no meaning for others, having it optional is a good thing.
And yes, this is a design issue. As there are many things that are part of design, but can have downsides with them. Crunching sounds when someone is eating, as a healing sound effect can be disruptive to some, while for some its immersion. Good design should enable to substitute these things with something that in general can be accepted. And its down to the devs on how far they are willing to go. Too much and some will call it dull/excessive, too little and it will keep players away. Or another FF related thing: the fireworks effects in any city. At some point its just boring audio clutter that disrupts music. And it doesnt matter if its ingame music, or a spotify playlist, it disrupts for absolutely no purpose other than 'immersion'.
At least, if it doesnt require a lot of effort, its in many cases worth to consider it. Especialy during reworks, its something that can be improved a lot, and maybe you can even use this for diffirent features as well improving the game overal. Small steps can have great results.