Jock, your last point in the original post is a very good one.
Even assuming that such devices don't interfere with aircraft systems, with the volume at which some people have their machines they'll never hear evacuation commands. If they're seated on an aisle they will then hinder other passengers. Further, having extra clutter on their laps will, even if they hear the commands, delay their evacuation at a time when seconds count.
On the basis of security alone, I am opposed to people having electronic devices in their carry-on baggage or on their person.