A third slip is a bet on your own bowler, not on the batter

Captains set the cordon by reputation and conditions when the honest question is how many balls will actually find the edge.

Ask a captain why he has three slips in and he will talk about the pitch, the new ball, the batter's habit of driving. Ask him after tea, when the third slip has spent ninety minutes doing nothing, and he will talk about the pitch again.

The cordon is the most visible statement a fielding side makes, and it is also the least examined. Each catcher back there is a fielder removed from the run saving ring, which on a flat day is worth something like eight or ten runs an hour in ones and twos that nobody remembers conceding. That cost is invisible. The dropped chance at third slip is extremely visible. So the incentives point one way and the arithmetic points the other, and captains follow the incentives, as anybody would.

Here is the part I think gets stated backwards. The number of slips should be set by how likely your bowler is to find the outside edge, not by how likely the batter is to nick it. Those sound like the same question and they are not. A batter's edge rate is fairly stable and pretty low. The bowler's ability to attack the top of off stump, ball after ball, at a length that draws a shot rather than a leave, varies enormously between the man who opens and the man who comes on first change. Put three slips in for a bowler drifting onto the pads and you have simply moved a fielder from a place he was useful to a place he is decorative.

The good captains I have watched change the cordon by bowler, mid over, sometimes ball to ball, and they take the third slip out the moment a spell loses its shape rather than at the end of it. That is discipline, and it looks like panic from the boundary.

There is an asymmetry worth stating plainly. An extra slip is worth most in the first fifteen minutes of a new batter's innings and in the first four overs of a new ball. Outside those windows it is close to superstition.

I would also stop calling it defensive to remove one. Nothing about a fielder standing where the ball actually goes is defensive. It is just honest.