ok, I agree that you CAN do without the mana casts.
However I'm a mage with just 10k Leadership and a wish to use oil mist and fire arrow as frequent as possible. Also my tactic require a stoneskin on my Red Dragons
I start combats with full mana which is 140. The skull makes it drop to 70. Right now Transmute is lvl 2 and concentration lvl 0, so if I don't use spells at all I will end the fight with 105 mana usually.
I can Easily do with 70 mana, but if I don't use mana accelerator, I will have to wait for the regeneration after each battle.
In a Montero Non-hero fight I usually get rage to cast 2 Mana Accelerators and 3 Treasure Searchers
I guess I could Call Colloseus for some Green Dragons. Except now its a Skull of Death

and I don't have the mana Issue any longer
Imo you need as much max mana as possible as a mage, and if that means -70mana in each fight with the Skull of Pain so be it.
The -20ish mana for a Paladin makes it a little less annoyong I guess
Edit: Does anyone know why my posts have spaces between each line? Only when I edit, I'm able to reove the extra space?