No, its not useless. Casting Darkness before Ultravision is perfectly valid, it stops you from attacking them immediately while they begin their other buffs, while helping hide any other shadows nearby who often stealth when returning to attack.
The effect of darkness does two things, it blinds creatures within, and it gifts concealment (that is pierced by ultravision instead of see invisibility), so as a protective spell it works well.
While attacking a creature that is concealed (not visible by Spot/sight to the player) does offer a 50% concealment anyway, there is a mechanic for being blinded by darkness which causes an "attacker miss chance" percentile of 50% to those blinded as well.
The Blind-Fight feat, rather than offer a reroll against this additional miss chance percentile, actually eliminates it entirely in NWN, so not everyone sees that or knows about it if they've always been told to take blind-fight anyway.