I've never seen "copying" or moving a user's comments from a page where that user placed them into another user's talk page as a "standard protocol" at all. Other than to "quote" an excerpt, with a link to the full writing, and to the location where it was originally written (so it can easily be viewed in its original context) is in IMHO extremely rude and very disrespectful of the original author.
The main problem is assuming that a "tacit agreement" exists, or that "whatever" somehow mitigates or excuses, the inappropriateness of manipulating another user's comments - that is an entirely different concept than "mutually editing" an article's contents. An author of a comment should be able to decide where those contents should be posted precisely to control the context in which his/her comments appear. Anything less than this kind of respect for a commenter can only have a chilling effect on a user's willingness to participate (and is one of the reasons I don't participate on wiki's more than I do).
Sure "comments" will "bounce back and forth between talk pages", as users visit and participate on those pages, but the user who posts them chooses (hopefully choosing carefully and respectfully) which page to place them on, and it is not proper for another user to "override" that choice on a whim or to support an agenda. It's easy enough to discuss it with the author first, with a comment on the page where the original writing occurred (or on its associated talk page).
As I'm sure you are well aware, I recently "agreed", after the fact, to such a reposting of my comments on the Dreamhost Wiki in a recent instance where a user insensitive to this protocol took it upon him/her self to "move" my comments from the page where I had originally posted them onto their own user page.
I removed them from that new page initially, leaving, in their place, an edit explaining that the comments had originally been made on a different page and that I did not believe my comments were relevant in the context of the page to which they had been moved. That user then "reverted" the page to replace my comments into the page, apparently believing they were important in the new context.
While I disagreed, I believe in standing behind what I say and/or write, and, since I had written what was "moved", even though I did not feel it was appropriate for them to be moved without discussion, and did not believe that the comments were relevant in the context in which the were now being "reposted", I agreed after the fact to refrain from getting into a reversion skirmish over the matter.
That "post facto" deference should not be construed to indicate that I believe the moving of the comments was proper; only that, in that case, to me, it was not worth arguing with anyone about .
I treat others' comments with respect and I don't believe it is my place to "edit" or "move" them from one page/article to another (though I do recognized there are "fair use" types of situations where limited "re-quoting", with attribution and linking, are appropriate).