They might have a 1-2-3 damage combo with MP steal and a 1-2-3 aggro building combo with a small amount of HP steal. Going into 'darkness stance' cancels HP & MP steal but reduces incoming damage and opens up various magic abilities.

The stance might not drain MP so much as the magic the job has access to is very MP heavy and the DRK can't get MP back while using it.

*DRK tanking away.
"Oh no, big hit incoming"
*activates dark magic mode, throws up dark shield.
*takes hit like a pro
*DRK uses magic life steal spell to recover some HP
"Oh no, out of MP, best leave dark magic mode so I can regen and steal more."

Sort of like that, but you know, better.