All very true and well said. The drifting into abstractness, where the impact lines are more of a symbol than an actual visual that we would see, is what bothers me about using it in my own work.
My stories, to my mind as I am writing/illustrating them, are being done in comic book form because that is the medium that is the most readily available to use and distribute in most cases. I don't imagine impact lines being on a cartoon or film, so I don't imagine them when I'm making the story in comic book format.
Having said all that, they are effective when used correctly, and accomplish giving visual information the the reader ( the dramatic force and impact of a given punch or such) in ways nothing else quite could.
There's give and take with any set of techniques you use. If you look at Wally Wood's work on EC comics (I'm gonna assume, by your in depth answer/understanding that you know who he is. if not I can find some examples for ya) He used no abstract devices like impact lines of motion lines, and his work then, while beautiful, was quite boring compared to the excitement in a Jack Kirby page.