It would be very nice to have day&night schedules not just for fluff but indeed with gameplay implications like special night content/quests, vampirism, stealth advantages at night, more dangerous exploration and so on...
But I can understand why they are not doing it. I'm not exactly a programmer but even from my limited understanding of how software or games work, I would guess that basically everything inside the engine would have to be changed to work "by the clock" or on a schedule.
For NPC schedules you could probably get away with some scripting but meaningful day/night schedules would demand huge architectural changes under the hood since every gameplay system would have to be put on a timer.
Since Larian are going to use the same engine it's probably just not feasible. I guess we're just going to have to keep waiting for that big RPG to dwarf them all
... in which we will hopefully have it all, day&night, NPC schedules and also seasonal changes/activities (I really miss games with changing seasons!).
