Doctor I am very much in agreement with you, the additional protection provided by a RCD on a selv circuit would be pointless.
However the rules need changing or an agreement to allow us to put it as a deviation from the regs, without some smart ass electrician who comes after you telling the circuit police.
My last assesment had a 12v fan and lighting in the bathroom, no RCD the assessor was ok with that but advised me to just comply with the regs until it was changed.
The supply cable ran up the height of the house in metal conduit so the 50mm rule never came in to it.
The shower was RCD protected.