Well from the perspective of an (ex) longhaul Captain, then yes it does happen.
Not very frequently, maybe 2% of lomghaul flights. Sometimes the galley areas get too cluttered with passengers after the meal service, and it is difficult for the cabin crew to get space to eat their own meal and they may ask for the seatbelts to be switched on to facilitate that. I don't consider it unreasonable for a short period of time, but wouldn't condone it for periods greater than 30 minutes or so.
Hope that helps....