Zenner Barriers: How They Work 

In my understanding a varistor would be used within an electronics circuit to protect against voltage spikes. Zener barriers are used to limit the potential power available to a remotely located instrument device which are located in hazardous areas to prevent enough energy to cause a spark which could result in an explosion.

Good point, as far as I am aware there would be no reason that a transistor with reverse bias could not be used in place of the zenner diode