I usually Tuck my reader away when the flight is taking off or landing, but on my last flight, there was quiet a delay waiting for a take off slot, and I kept on reading, only realising as we levelled off (Yes The book was that interesting, but is was more that I have waited thru too many take offs and landings that I just tune it out)
Anyway, I haven't been asked by cabin staff to switch it off any of the times I have continued to read, even when I was once in the first row, just in front to where they were sitting.
The only query I have usually had is of interest, of what it is, and where to get it. I just point them to this site, to get the best info
Seriously, from what I understand there are two main reasons for being required to switch off electronic devices. The first as mentioned above, is a question of interference, however remote of a chance it might be.
The other is that the passenger is alert, and able to react and follow instruction in case of a emergency. after all you don't want someone so engrossed in their music or playing a game, etc. that they can't follow the crew's instructions.