What happens if you only energize one pole in a 2-pole 240 V circuit?
You are correct. The circuit is not complete if one line is open. However the load is still hot to ground. If someone gains access to any part of the circuit, they could get an electrical shock. If water or vermin get in, there could be a short circuit to ground. The circuit breaker should prevent a fire, but it is still somewhat of a hazard if something happens when no one is around. It is better for both lines to be disconnected when the power is supposed to be shut off.