It makes sense to turn them off. It is easy to demonstrate that digital mobile phones can induce interference. Put one next to a Sigtronics intercom and it will be obvious when it is looking for a base station by the chirping you get in the headset. This type of interference is no more than a distracting annoyance and is unlikely to upset the simple analog systems in an older aircraft beyond a temporary blip. It sure aint going to stop my magnetos working.
Digital circuits on the other hand may interpret a segment of the interference as valid digital code and act accordingly causing unpredictable results! Properly designed, screened digital circuitry should reject this type of interference , but it can and does cause problems. I have found early digital medical equipment is easy to upset with a mobile phone so I suspect old digital designs are most at risk.