If you have Devil's Sight, Darkness have no effect on you (as far as i understand it) unless there is something else (like fog mist, for example)
If your oponent have Devil's Sight, the story should be the same ...

So duel between two Devil's Sight Warlocks, would mean just wasted spell slot with zero effect, by using Darkness. laugh

I still dont understand why cant we change Race for our hirelings. frown
Lets us play Githyanki as racist as they trully are! frown