Sorry, but I was not questioning your ability, I was just trying to make sense of what the lane assist is trying to do, and with it switched on, unless indicated, it will try steer drivers back into the lane as it thinks they may have strayed by accident. One such scene that I think you are trying point out is when you are coming off a slip road onto a major road, no need to indicate if the major road is clear, in that case, I totally agree with you. I have not had blue light training but I have been a professional lorry driver and covered many hundreds of thousands of miles during my time.
As to saving fuel at idle, when I was not paying for fuel, I also to think it didn't make much difference, however when I started to pay for own fuel, I did notice the difference, I suppose it makes a small difference in may be more rural areas and a far difference in cities like London.