This is actually quite the opposite of my fairly substantial experience with the London squatting scene. The vast majority of them seem to be well-meaning, anarchist or at least hippyish artistic young people. Most of them don't smoke or even drink very much. They're very good at caring from their property, especially historic buildings. One group I know paid for an expert to evaluate and do some repair work on some antique wallpaper, which had originally been left to rot by the absent owner. The friends in commercial spaces will keep them tidy, clean and free of vermin. They use the space, as opposed to letting it sit, rotting and empty in the heart of a city on the cusp of a genuine housing crisis. They put on workshops and host events, trying to bring life to often wholly non-residential areas. They make themselves part of the community. As far as I have seen, they're a wholly positive force for good.

Here's a counter-example to your article: http://londonist.com/2009/01/temporary_school_of_thought.php