When is a mixed strategy appropriate?

When only player $($A$)$ has a dominant strategy, and the other player $($B$)$ doesn't think player A is predictable.

When both firms have a dominant strategy, and the Nash equilibrium is a prisoner's dilemma

When both firms have a dominant strategy, and the Nash equilibrium is not a prisoner's dilemma

When there is no dominant strategy for either player in a game played repeatedly