Do these have any sort of legitimate use? Does it extend past the normal +35 cap? Otherwise who on earth would use 300 class excubes (realistically 600 normal excubes) to do this? 13*s already come with cap extended to +35

They were originally added to the game back when 13*s could drop with a low Grind Cap, and they were left in the NA version because technically they still have a use, even if the cost is wildly inefficient.