I wouldn't trust the Osu! difficulty rating system that much, it is not that reliable in most beatmapper's standards (it really is there just to give the player a hint though).
Otherwise, I think the insane difficulty is only applicable if the beatmapper intends the map to be impossible to beat. At that case, and here I am only speaking to my own standards, I feel as if there should only be one difficulty in a beatmap that should be impossible to beat, not two.
Of course, again, this is only to what I simply have seen from my time in Osu! Whether you want to take it in or not, that's your choice.
Otherwise, I think the insane difficulty is only applicable if the beatmapper intends the map to be impossible to beat. At that case, and here I am only speaking to my own standards, I feel as if there should only be one difficulty in a beatmap that should be impossible to beat, not two.
Of course, again, this is only to what I simply have seen from my time in Osu! Whether you want to take it in or not, that's your choice.