One fine day I was in the back seat of an L-19 on a glider tow. After release I was flying it back. All went perfectly fine until turning final with flaps down:
the stick came out
the nose promptly dropped and the pilot in front exclaimed and took over.
With things mechanical failure does occasionally happen, whether controls, seatback or prosthesis.
In many tandem taildraggers, a stick is removable. Of course that exposes the danger of the stick coming out at inopportune times. Following the preferences expressed by some here, the stick should be welded in to preclude that failure.
However seatbacks break. If one falls on the rear stick and there's no rear seat occupant, the usual result is a crash with serious injuries or death.
Thoughtful risk management requires consideration of all factors.