I can at least understand why sometimes people think it unnecessary. The problem is that it then becomes a habit. If you allow people to use discretion as to where and when they bother to indicate you are always going to get some **** who regularly makes bad, inconsiderate and potentially lethal decisions.
So it is best to just do it all the time.
As the sub-human filth that I am, I once didn't signal at all when going right on a roundabout, not the right signal as I turned or the left signal as I exited. Simply because I was in a foul mood and couldn't be bothered.
Turns out the bloke waiting to enter was a proper road rage nutter and he let loose a right volley as his eyes bulged out of his head.
Most amusing I think I might do it more often just to see the nutters react.
It's not actually etiquette, though, is it? Indicating is actually road safety and basic driving competence. If you don't indicate when turning, you are an unsafe and incompetent driver. What is more, you are almost certainly likely to be in breach of the law.