I disagree quite strongly. You have to remember that XP for users is not important. Votes for nodes *are* important. If plagiarised nodes have got good votes, then that is an indication that the nodes are good. They should keep their votes, so that other users can see "ooh, lots of + votes, I'll pay attention to this good advice". Doesn't matter if it was copied, it's still good content.
I suggest that if you think a node is plagiarised and if you can be bothered, notify the author it was copied from. They can then post a note in the thread pointing out where it was copied from if they care. Getting heavy will be pointless as they can just create new accounts and will probably whine and bitch and generally annoy people.
As for the suggestion that others have made that they've probably done this to get a good reputation - well, it hasn't worked. I'd not heard of either of 'em until this thread started and I'm sure I'll have forgotten their names in a few minutes. At least for me, posters' reputations here depend not just on what they post here, but also on their behaviour and posts in other places.