Ok you guys, since this has been posted 4 times, there's obviously an appetite to discuss it, so I've turned off the user flags on the original post and merged the comments thither:
In cases where we're merging thread B into thread A, it's not in anyone's interest to have both A and B hovering in the user's face, so once we've moved all the comments into A, we apply the maximum downweight to B.
Does that answer your question?