As for indicating, like various people have said, I don't think they even really say you mustn't if nobody is around. But you would be challenged on why you'd done it if there wasn't anyone to benefit.
The idea was, again as said, to remove habitual unthinking behaviours when conscious ones would serve everyone better. You shouldn't consider yourself infallible but if you were any good at driving then situations where you didn't observe a hazard ought to be extremely rare. So the habit ought to be looking, identifying the need and then doing something about that.
If you were in a busy urban environment then the answer should be simple - someone may benefit. If you were exiting an apparently empty motorway then either nobody would benefit, or you have fucked up big time on observation and situational awareness, sort yourself out.
So, if you wholly subscribe to or reject this theory, which you don't exactly have to do, then the choice is really between 'don't ever fuck it up', and 'always give people clues just in case you do fuck it up'. The IAM probably thinks you should be able to do the first one, and I think they're basically right. And giving people clues is no guarantee they'll see or benefit from them, leaving you with 'but I was indicating!' twattery.
I think people take this one a bit too seriously though, and why not, it's classic Internet argument material.