Yeah - i suppose there's a line there. Do people really sit down and learn the harmonies like that? Sounds a bit like disecting the song, which would kill it for me - and then if people copy the whole arrangement with complex harmonies reproduced exactly they might just as well put the CD on.
I think I just failed to get the point of what you are saying because that way of doing things is totally alien to me & so I don't really understand the work involved either in deriving a new arrangement or copying an existing one.
I'm still not completely convinced, but then perhaps I'd need to know more about the subject. Seems to me though that it wouldn't hurt to acknowlege the debt where there has been a concious attempt to copy an arrangement. Don't know whether I would assume that a performer had derived the arrangement themselves in the absence of an acknowlegement. Will give it some more thought.