I have done it many times per EE instruction, though it is not required, I think the EE did it this way for cost efficency. The main breaker was a shunt trip type, and was used to drop the building load when the pump was running.
I guess one of reasons we do this is when we can't coordinate between EG main breaker and fire pump breaker, I am not sure you can drop ALL building load even when fire pump runs (i.e egress lighting, fire alarm, etc)
