Not a massive fan of nightwatchmen. If he fails, then you achieve nothing as the batsmen is exposed anyway. If you use 2 nightwatchmen then you're going to significantly increase the chances of your number 6 or 7 batsman being stranded when they're actually batting at 8 or 9.
Best case scenario is if he succeeds, then the nightwatchman almost always ends up being an easy wicket the next morning, so you just end up giving the opposition attack the morale boost of picking up a wicket.
However, using nightwatchmen in video games is great fun trying to get your #11 to a test century, and should be encouraged at every opportunity.