Rather than - or at least in addition to - being a bug, this strikes me as one of charity's features. Most arguments are, indeed, neither original nor very good. Inasmuch as you can substitute them for more original and/or coherent claims, then so much the better, I say.

Yes. But it's not doing any favors to anybody if you pretend that a new coherent argument is the same as an old incoherent argument. In my experience, the authors of the previous argument are often hesitant to agree with the new rephrasing -- it's not written in the terms they use to understand the world.