In my limited exposure to Android, I've been told that Android shuts down unused apps to make room for whatever app you're using. Well-written apps will work seamlessly with Android to shutdown when requested, and reopen to their last state when the user switches back to an app. Some apps, like email, tend not to shut down because they're always doing something in the background (i.e., checking your email account). Most non-Internet-connected apps should shutdown politely, though, making Taskillers only necessary for ill-behaved apps, or apps/services which insist on running every time the device boots whether you want/need them or not.

That's the theory as I've learned it, anyway