In defence of the nightwatchman
The most mocked call in captaincy is usually right, and the mockery comes from people who have never had to bat at twenty to seven.
A wicket falls with fifteen minutes of the day left, a number nine starts putting pads on, and somewhere a commentator sighs. Negative. Old-fashioned. Wasting a wicket. The complaint is louder than it has ever been and it is still mostly wrong.
The argument against, taken seriously
It goes like this. You have eleven batters and you have just chosen to spend one of them on twelve minutes of survival. If they go early you have lost a wicket for nothing and your proper batter walks in anyway, with the light worse and the crowd restless. And if they survive, you start tomorrow morning with a tail-ender on strike against a fresh bowler and a hard ball.
All of that is true. It is also priced against the wrong alternative.
What the last twenty minutes actually are
They are the worst batting conditions in the match. The light is going, the bowlers know there are only three or four overs in it so they can empty the tank, the field is up, and the batter walking out has been sitting in a chair for two hours. Nobody makes runs in that period. The only thing available is not losing another one.
So the question is never whether the nightwatchman might get out. It is who you would rather have facing those overs, given that somebody has to. Your number three, tomorrow morning, fresh, against a ball that is forty overs old, is worth a great deal more than your number nine is worth at any point in the innings. Protecting the more valuable asset in the more dangerous conditions is not sentiment, it is arithmetic.
What actually goes wrong
Three things, and none of them is the tactic.
The first is sending in somebody who cannot defend. A nightwatchman needs one skill, and it is the forward defensive. If your number nine is a slogger who has never left a ball in their life, you have not protected anybody, you have just changed which wicket falls.
The second is sending them in too early. With half an hour left, there is real cricket to be played and a genuine batter should play it. The window is small: enough overs left to be dangerous, not enough to be worth batting properly in.
The third is not telling them the job. They are not there to score. They are there to see out the day and then, in the morning, to hang around for as long as they can while somebody else scores. A nightwatchman who plays a drive in the fourth over of the next morning has misunderstood the whole point of being out there.
When it is genuinely daft
Any game with a fixed number of overs. If the innings ends by the clock rather than by wickets, a nightwatchman is spending balls you needed. Save it for cricket where the innings can last as long as you can make it last, which in club terms means two-day games and declaration cricket and very little else.
And if your number nine is having the summer of their life while your number six has not passed twenty since May, send the number six. The roles on the scorecard are not the point. The question is only ever which player you would least like to lose in the worst fifteen minutes of the day.
The short version
It is a decision about who faces the worst conditions of the match. Get the right player, send them at the right time, and tell them what the job is.