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.