Adding a day/night system means you have A LOT of variables to account for. You would have to add complex behaviors to otherwise pretty mundanes scenarios. Imagine being in a big town having to sync about 50 npcs in real time to make them behave logically to the current time with multiplayer in mind. This would technically be doable, but the scope creep would be insane and we aren't even sure the underlying engine can handle that much. I would too like for every games to be as deep as Dwarf Fortress, but the cost vastly outweigh the benefit there.
What you're actually talking about is "adding complex, realistic schedules for all NPCs that depend on constantly passing time of day." However, having such NPC schedules is not a requirement for implementing a Day/Night system. A very simple Day/Night system is two have two states - day and night - which change instantly (likely during a loading screen). Certain NPCs disappear during night time, and certain NPCs only appear during night. Or as @Rag suggested above, don't even bother changing NPC schedules.
@Rag, the multiplayer part *potentially* comes into play when you consider that some players can be in TB mode while some players aren't, so NPCs & times for different locations could get off-sync with each other. From Larian's response, it seems like they're not willing to consider a imperfect system of time passage, and thus are ignoring the countless of simpler solutions like:
- tie time passage to a the host of the game
- if at least one person is currently in TB mode, have time pass according to that slower schedule
- don't use constant time passage, but instead have discrete day/night(/afternoon?) states
- etc
Perfect is the enemy of the good.
