This spell is excellent for an Arcane Warrior tank. The defense penalty applied to enemies helps a lot; Arcane Warriors often struggle with low attack score. And the extra threat (when on Hard / Nightmare difficulty) is welcome.
Miasma's speed debuff can be stacked 3 times.
Miasma will allow you to see a cloudy version of enemies in stealth, as long as Miasma is not resisted.