Blacksheep, you're right, in that an announcement regarding evacuation is unlikely to occur in the cruise.
However, it is quite possible that an urgent safety announcement needs to be made. In that event, I would want to know that everyone has heard and understood it.
Since the aircraft's PA overrides the sound on all IFE systems, you may conclude there is a reason for it. Furthermore, given the range of choice on most IFE's nowadays, I see no reason why people need to listen to their own PED's, at least while on board.
Or does safety no longer come first?
As for "leakage" from aircraft avionics systems, granted that you can't make them all leak-proof. However, the manufacturers go a long way towards trying. The manufacturers of PED's don't even try. Another reason for banning them on board.
There is less reason for banning use of laptops in the cruise. Business travellers (and others) will often need the time to work during the flight, they don't block off the hearing, and their use is very much more obvious to the cabin crew than a PED, so they are more likely to know who to approach to have it turned off if the aircraft's systems start keeling over sideways. I do, however, feel that the cabin crew should be notified before pax use them - gives one a fighting chance!