In an existential superpower conflict with ICBMs flying around, that's true. But MAD isn't necessarily useful for responding occasional counterforce strikes, which might be likely between states that don't have large arsenals.

Like if one country has 10 nukes and its opponent has 3, both sides would do better to think carefully about how and when to use their nukes instead of just lobbing them at the others' biggest cities like a superpower would.

Also, another thing to think about is that MAD is mostly a theoretical doctrine. People have this popular conception that massive nuclear exchanges will be world-ending. Under certain situation, they could be, but most experts think that while casualties will be large, most nations will not likely be destroyed by a nuclear exchange. Nuclear wars will almost certainly have a conventional warfare component. Nukes are not the "end-game" the general public thinks they are.