Google doesn't like task killers, and they've been steadfastly saying so since the release of Froyo - which, not incidentally, broke functionality for many of them. There are still some hold-outs that claim they're useful for saving the ever-important battery on Android phones, but according to some in-depth research from PCWorld, that just isn't the case. After running the most popular example, Advanced Task Killer, through its paces, the conclusion was that the extra monitoring and other services had almost no effect on the phone's longevity.