At risk of repeating myself, as this topic has already been done to death on
this thread, anybody who uses unauthorised electronic devices in flight is probably breaking the law and could potentially be charged with an offense. Furthermore, failure obey the instructions of the crew is also illegal.
For those of you who read this and decide to continue to flout the regulations, I'd guess that the judge wouldn't accept the excuse that "someone on an internet blog said that interference isn't actually an issue." As this
Flightglobal Article shows, electromagnetic interference is an ongoing issue. Note how in this case, Wi-Fi (and possibly cellular) signals adversely affect one particular version of Display Units from a major manufacturer, (display units are the screens which display instrumentation, engine, navigation, and other data on modern aircraft to the pilots).
The rules are in place for the protection and safety of the aircraft and all of its occupants. Whilst you (SLF) may find it an inconvenience to turn off your devices at the specified times, trust that a lot of work does go in to allow you maximum freedom to use your devices. Unfortunately no-one can currently ensure that certain devices will not have effects at times that adversely affect the safety of the aircraft. In the aviation industry, safety is always paramount, end of story.