Yes I agree, If the lights or shaver sockets in the bathroom are class II or out of simultaneous reach. I was thinking along the lines of a snagged cable loose above the bathroom making fortuitous contact with the plumbing pipes as stuff gets moved about in loft spaces.
I chose a lighting circuit really to demonstrate that an earth fault voltage sitting on extraneous metalwork does not have to be 0.05 ohms, the largest Zs allowable which would keep the touch voltage to a maximum of 50V. If you had a 10.8kW instantaneous water heater which had caused an earth fault right above the cast iron bath you were standing in while having a shower then you would need a theoretical resistance path to the MET and around the bathroom to be no higher than 50/250 = 0.2 ohms. (personally I would prefer in this case the bath to be isolated from the MET and surrounding metalwork)
Now that RCDs exist and a global 0.4s disconnection time on final circuits, the importance of 'front line' protection by the use of supplementary bonding has lost its importance.
But we still have uncontrolled conditions out side of the installation such as network fluctuations in voltages, lost, damaged, or high resistance supply neutrals, which can lead to voltages appearing on simultaneously touchable metalwork for longer periods than 0.04, 0.4s, 5s. Voltages from PME systems could be there indefinitely until someone with wet hands and feet comes into contact.
