I would say that it is because the way electronics works today, the PA system might be down while listening to music. The best example is the PA system working and the movie not!! Now imagine the reverse!! Then an emergency occurs during the landing procedure. How do you adress the pax onboard, specially those with the headsets still on?

We should never take flying for granted and always respect the rules, they are there because something happened in the past that may have caused some serious problems, sometime crashes of aircrafts...

I'm not an expert by all means, but as far as I'm concerned, these rules have an origin that might have cost lives in the past...