My personal favorite is all the "memory" information in most task killers. I commonly see a giant list of apps running, each using slightly over 17MB of memory. Of course if you kill one, you only get a few kilobytes back, not 17MB. Why? Because it's mostly shared memory, i.e., every single app is sharing almost all of that 17MB, most apps probably have 1-2MB of data at most. Things like the JVM code are included in that 17MB, even though only one copy of that data actually has been written to memory, and is shared amongst all applications.
If you find you're experiencing poor battery life due to running applications, find out which applications are doing it. Don't kill them, uninstall them, and send the developer a message about it. Check the battery usage application.
I do like the idea of having a task killer to stop bad/broken applications though. If the problem is a consistent one, my next step tends to be to uninstall the app though.