This is a brief extract from this article, explaining why we should no longer use Task Killers.
Hello, friends. This is an Android Public Service Announcement regarding the overuse of “task killer” apps in the Android community. Almost everyone has experimented or used one of these apps at one point in their Android life. I wont judge you. In some cases, these apps might have been installed even without your knowledge. I know for a fact that reps at carrier retail stores will download and install these upon buying any of their Android phones. But why? It’s largely in part because of this HUGE misconception that Android needs these apps in order to run properly. But this couldn’t be further from the truth.