It's not like the item reduces the effect of on-spell-hit effects like Rylai's which is a great pick on most of the mages listed above or any built in side effects of spells. It's flat damage reduction to give tankier champions a better chance against the things AH was designed against.
DPS mages are usually either way in the back of a fight or in the midst of it with bulky AP items such as RoA, Rylai's, or Abyssal Mask which halves the effect of Adaptive helm. CC isn't all that punishing to them either compared to their AD counterpart ADCs as they already dealt their damage by the time CC hits them.
It doesn't completely fuck them over, it just gives them something to take into consideration.