Dark magic, and in particular, general spells, allow,
firstly, to somehow justify the huge amount of mana (some of which is clearly superfluous),
secondly - the effectiveness of the hero's turn is increased. A small witchcraft parameter will not prevent casting very useful general spells.

And attacking with Chaos is fundamentally wrong. You can simply ignore the demonic, chaotic, ridiculous damage.

Even the Fire Lord (considering the ability to cast a devastating beam as a general spell, and not just for one turn) does not justify the use of Chaos demons.

So I wouldn't advise anyone to level up Chaos for demons.

I even tried to somehow justify it, but nothing comes to mind, except perhaps strengthening Hellfire.