The slice is only defensive if you hit it late

A backhand slice taken early and flat is an attacking shot; taken late and high it is a request for mercy.

The slice has a reputation problem. It is filed as the shot you hit when you are in trouble, which is true of one version of it and completely wrong about another, and the difference is almost entirely about contact point.

Late slice, taken beside or behind the body with the racket dropping under the ball, floats. It has to. The only way to get a ball over the net from that position is to open the face and add height, and the result sits up, travels slowly and gives the opponent a stationary target at a comfortable height. That is the shot that earned the reputation.

Early slice is a different animal. Contact well in front, racket path forward more than down, and the ball leaves flat with backspin that makes it skid rather than sit. It arrives fast, stays under the knee, and forces the opponent to hit up from a low position, which removes their ability to attack and often produces a short reply. On grass it is close to unanswerable. On a slow hard court it is still a good way to break the rhythm of somebody who wants everything at hip height.

The reason players do not have it is that it demands early footwork and there is no way to fake that. A topspin backhand can be rescued with the arm from a poor position. A driving slice cannot. So it is a shot that punishes exactly the players who need it most.

I would add a further complaint about how it is taught. Coaches demonstrate the slice as a soft shot in a warm up, standing still, a gentle rally ball. Then they wonder why students only produce it under duress. Teach it as an approach shot, with a step through, and it becomes a completely different proposition in the student's head.

There is a fitness angle too. The slice is cheap to hit. In a long match, a player with a real slice has a way of playing a neutral or even aggressive ball without spending anything, and that is worth several games late in a fifth set.

The players who cannot slice are not choosing not to.

They just never learned to arrive on time.