The slice is only a defensive shot if you hit it badly

A low backhand that stays under net height forces an upward strike, which is the least comfortable ball in the modern game.

Watch a rally where one player keeps chipping the backhand and listen to the commentary. He is described as being pushed around, hanging on, buying time. Sometimes that is true. Often the player doing the chipping has taken control of the point and nobody has noticed because the shot travels slowly.

The mechanism is about height at the bounce, not about pace. A modern topspin forehand is built to be struck between hip and shoulder, with a low-to-high swing that turns the incoming ball's own bounce into pace. Feed that swing a ball sitting at knee height or lower and the entire kinetic chain is wrong. The player has to drop the racket head under the ball, generate everything himself, and hit up over a net he can barely see past. The margin collapses. The pace he sends back is his own, which means it comes back at him with nothing on it.

That is why a good slice is aggressive. Not because it hurts, but because it removes the opponent's ability to hurt you and gives you a slow ball to step inside on the next shot.

The reason it looks defensive is that most players hit it badly. A slice that sits up at waist height is the worst shot in tennis, a free attack handed over with no pace on it. A slice that floats long is worse. The difference between the good and the bad version is entirely in how flat the racket path is through contact and whether the ball skids rather than bites, and getting that repeatable takes the sort of practice hours that nobody wants to spend on a shot that does not appear on a highlight reel.

I would argue it is the single most undertrained shot in the junior game. Coaches build two big groundstrokes because two big groundstrokes win under-sixteen matches. The slice gets treated as a survival tool for when you are stretched.

Then those players reach a level where everybody has two big groundstrokes, and the ones who can change the height of the ball start beating the ones who can only change its speed.

Low and skidding is not a compromise. It is an instruction the other player cannot refuse.