The day/night cycle in the original BG I & II was very important for characters such as thieves, rangers, and monks with Hide in Shadows abilities. I don't know the numbers, but I suspect night stealth activities were twice as likely to succeed ... huge bonus!
Day and night cycles can actually do a lot for a game, just as indoors versus outdoors, on an emotional level. Entering a forest with the rays of dawn filtering through the trees, climbing up to a fog-shrouded temple on a moonlit night, etc., can all set an emotional tone for an adventure. A picture is worth 996 words (we just lost a few due to inflation).
