Shutoff Between Expansion Tank And Backflow?

[FraserJim]

We have a radiant heat system with a boiler, zones, baseboard, and DHW. The backflow preventer started to leak. The shutoff from the city supply is always closed. A plumber came in and told me we needed to have a shutoff installed between the backflow preventer and the expansion tank. Does this sound right? The only purpose, that I can see, would be to service the backflow preventer.