Plastering is done on any surface to increase the durability of the material on which plaster is done.
The structures are designed with assumed life span of years.
Decomposition of the material used to form a structure reduce the serviceability of the structure or need early maintenance of the structure to keep it in service.
A wall which is non-structural with low life expectancy may not require plastering on it. Plaster act as a weather coat which protect base from weathering.
if the base is weather proof or more durable and can meet the life expectancy then it will not require a plaster on it.
In such cases brickwork shall be done in such a way that, it wont allow the seepage of water inside of the room or stagnation of the water on external surfaces.