When I asked the plumbers installing my new boiler why they did it, they said that their electrician said they had to!
I guess these are the same type of electricians that specifies that the gas engineers replacing the gas mains have to use a non-contact voltage detector to check that a consumer's pipework is not live before re-connecting them after putting in a plastic pipe, and then when the (bonded) gas pipes and the MET show up as live (due to capacitive coupling from adjacent mains wiring to the gasmans' body) refused to connect. Had to get the DNO out, who told them it was all OK and that their volt sticks were a pile of sh!te. Happens all the time, the DNO guys said.
