The rule about breaking down a cue is mostly directed at the shark move of a player breaking down his cue while it is his opponents turn. As when your opponent is shooting the money ball or has an easy pattern to get out, many players will try to put more pressure on their opponent by making a big show out of breaking their cue down. They are making the statement that it is soo easy that you couldn't miss, and want to try and hurry up the player or cause them to make an error. It is a last ditch effort and is considered unsportsmanlike conduct.
Notice the rule actually stipulates that it is a forfeit if the player even ATTEMPTS to break down his cue. You do not have to even disassemble the cue to forfeit.
We used to have a house rule like this apply to the opponent grabbing the rack and holding it over the table or swinging it about, etc. To me it is the same thing, I wonder why the rules do not have something like this as well?
Jw