Oblivion, Skyrim, FO3/NV all did this: NPCs would sleep at night (or go about their more nefarious business), stores would close and so on...
Boycotted the last two (Steamworks exclusives) but Morrowind could probably qualify also.
...think both BG2 and Elder Scrolls : Skyrim did a great job.
BG2 didn't implement general NPC day/night activity, you just had a few key encounters limited to nighttime.
...But even a cosmetic day/night cycle adds to immersion and opportunities for roleplaying. Putting the player in a game world which is perpetually bathed in daylight is about as immersion breaking as you can get. And for those who like to play as thieves, well, no night is a serious issue.
On the other side of the coin, having a merchant standing out in the open regardless of weather or time of day can be just as immersion breaking (glares at NWN2).
There are, however, some fairly simple ways to add strategic depth and the illusion of NPC schedules to a day/night cycle without having to actually code it in completely. Closing shops at night by simply locking the doors...
Such additions would be welcome, but can hardly qualify as simple unless you're prepared to accept immersion-breaking shortcuts. In D:OS most shopkeepers have outside stalls, so closing them would mean having to teleport the stalls (or at least the items on them) in lieu of scripting each merchant to pack up. And most merchants don't have homes so they would have to vanish until morning.
In the case of buildings, what happens when closing time occurs while the party is still inside? Unless a "we're closing, out you go"-type script is added to move the party out (and this needs to be individual to each building), then they risk being locked in till morning. Plus each inhabitant would need a nighttime script (a generic "Get stuffed! I'm sleeping! *zzzz*" should suffice in many cases though).
...Hopefully the game is successfully enough (one week Steam #1 sale) to giv'em enough space/time to reevaluate some decisions made in the past
We can hope, but it would mean a lot of re-engineering and re-balancing.