Earphones on cyclists: Yea or Nay?

I often listen to an audio book in my left ear - is that any different to chatting on a ride. OK, maybe a little - but you can listen to other things and still hear other traffic.

Loud music I would say no, since I want to hear cyclists if they want to warn me they are passing.

Not noise cancelling earphones, obviously, but I reckon I am more alert when I am not completely bored on a very long ride.

I don't. In both the forces, and martial arts training, you'll always be advised never to wear hoods due to the impairment of your hearing and the risk of attack from behind. The same applies to earphones. Whether that risk is from an individual intent on harming you, or as per this discussion a careless driver, makes little difference.

Maybe my background just makes me paranoid.

I choose not to i did a couple of times but the whooshing sound of the wind got on my nerves and to be honest taking in the sounds around me is one of the pleasures of cycling.
Can't sit on public transport without them though.

I don't, but if it doesn't impair your ability to hear what's going on around you I don't see that there's any harm in it.

It is frowned upon by those cyclists who have the ability to levitate, as listening to music stops them being able to work out when someone is about to deliberately ram them from behind. If you can't tell if you're about to be purposefully driven into, you don't know when to start the levitation process to avoid this.

I wear them, but I keep it low and only use the ones with ear clips that sit outside the ear canal so I can still hear traffic.

In winter I wear them just to stop the wind from chilling the inside of my ears too.

I listen to music, not too loud as to drown out the traffic though - so I can hear cars coming up still.

I don't (though on quiet mornings, I often do listen to podcasts in my left ear only).