Battlecry twice outvalues Hagatha . Hagatha is not needed

I just watch a video for this card . You are right . If I have 3 5 8 cost spells in my hand , it will summon 3 cost , 5 costs , 8 costs minions for total 3 minions . This one is insanely good with Shudd Shaman.

I think you misinterpret this . For example : if you have 5 spells in your hand , when you summon this , you summon 5 cost minion .