I miss the clock (I spent so long in the 2nd Mission that it went from 2-3am to dawn, and I wasn't expecting it! XD) but I have yet to feel inconvenienced by playing missions at night. My squad focuses on Sniping (though that'll change, eventually) so they never have any issues with accuracy, meanwhile I can RUN a few dozen feet past enemies and they won't even notice my guys because of the darkness, but I would get peppered with bullets if I did the same with light out.