People think that they need every bit of ram freed up on the phone or the battery goes quicker and the phone slows down. So they use task killers to constantly kill "running" processes they aren't using. It's the windows mobile way of thinking.
Android leaves processes in memory so it can open them up quickly next time you try to use them, but they aren't actually running (unless they are a service and are intended to be run). They just reside in memory, which takes the same amount of battery as if the memory was empty. I've actually seen some people claim better battery life when they did an uninstall of their task killer, which makes sense because these task killers are meant to be run as services, thus always on.
Now there may be some poorly written programs that kill battery or don't suspend gracefully, but there is a built in task killer for that in the android settings menus. Android is smart enough to suspend processes that aren't in use and when they go off screen. Which makes me think that people who claim Live Wallpapers hurt their battery are just guessing. The wallpaper shouldn't be running if you have an app up or when the phone is suspended, so it only runs when you see it. Obviously that takes juice more than when a static background, but how long do you just stare at your homescreens?