OK Graham, I'll allow this political rant just the once mate.....
Because, it's also one of my pet hates.
It's a puerile practice does nothing to enhance the perception of intellect and it is a thoroughly negative tactic.
Another interesting thing I learned from business is that on the principle "there is no such thing as bad publicity" merely mentioning the oppositions existence, means you have identified your "boogyman" as the one you most fear and that knowledge can be turned against you.
The black art of 'comparative advertising' started in the USA and is heavily used/abused. Here, I believe our Libel Laws prevent us from putting our snouts in that trough, and thank goodness for that. Politicians on the other hand are protected by 'parliamentary privilege' , stop that and you will stop the rhetoric.
I believe both comparative advertising and the automatic political 'smearing' of the opposition have their roots in the same principle - they believe they are talking, or relating to a dumb & dumber audience, and that, I find insulting.
