It would be a jarring inconsistency if you ask me. A combat round is supposed to simulate a handful of seconds of "real life", while what happened in real time to a player not stuck in combat takes minutes, if not hours. With a day/night circle in place this issue (which already exists in the first game) will be even more obvious and ultimately annoying.
In real life, travelling anywhere interesting by foot would take days, not minutes. Forests and towns would be much bigger, and you probably wouldn't have to stab someone more than once to kill them...
It's all fairly abstract.
Actually, the only time related abstraction in a turn-based combat system is the turn order. As for the time length of the rounds, they were traditionally supposed to represent a very specific and very short amount of time (usually from 2 to 6 seconds).
In a single player game, this remains consistent because the whole word freezes when a combat begins. In D:OS it doesn't happen (for obvious reasons), but at least, with no day/night circle in place the issue is masked to some degree, because the whole notion of "flow of time" is not in the game.
With a day/night circle in place it would become painfully obvious that something is wrong in the way the time flows.
