I'd love for darker nights, assuming NPCs didn't suddenly get ultraviolet eyesight as soon as you engaged them, and could see your character as soon as you left sneak mode just as good as during day.
It's a shame really, given AI in quite a lot of games simply knows where the PC is and don't have to struggle with seeing through the eternal darkness of the night.
I agree, I think the lighting technology and NPC AI is what prevented this portion of gameplay from happening.
I'm not a programmer but I would assume they could shrink the NPC vision to match the players average view distance during the specific time (day / dusk / night) and have exceptions for people with torches, racials, or spells.
I think day / night would be a great feature for both roleplayers and gameplay alike.