Interesting article on why we shouldn't use Task Killers on Android
Not that I completely agree as per my observation, but interesting to read - especially the section which mentions:
"On Android, having your RAM nearly full is a good thing. It means that when you relaunch an app you've previously opened, the app launches quickly and returns to its previous state. So while Android actually uses RAM efficiently, most users see that their RAM is full and assume that's what's slowing down their phone. In reality, your CPU—which is only used by apps that are actually active—is almost always the bottleneck."