Katarina need mana
I think every mid champion must have mana or energy and
above all. It's not fairplay for other mid champion who have mana. With mana,
won't be able to use a full combo 3 time in a row and she won't be able to spawn is attack to farm. Thats why mana exist. She is a mage so she need mana. If you that it will destroy the champion, think about other midlaner like
or
. What if we remove their mana. They will spawn their attack and it would be very very annoying. Exactly like
. Same thing for
, he need energy.