Hi 1st rate roofer,
I couldn't agree more with you. I almost always replace flashing. Roofers who nail flashing to the wall have their heads up their a$$. One nail per step flash into the roof. thats it. On vinyl siding jobs I often will just take the siding right off cause its easy, but on hardie plank or masonite it sucks. Cedar is ok.
The main thing is that often where a roof meets a wall there are seperate sections of foundation holding it all up. You nail your flasing to the wall and the footing settles in the wrong place and you have a leak.