Task killers/managers are not exactly the satan that some posters make them out to be. For example, you may leave an app running in memory using the GPS which is draining the battery. You see the GPS is currently on. If you're not sure whether its Maverick, Oruxmaps, My Tracks, Endomondo, Google Maps, etc... then you can bring up your taskkiller and quickly find out who is doing the damage. I surely don't want to wait until the app shows up in the "apps using the most battery" list.
I read recently a very frequently posting poster say they reboot their phone daily to make it run better, yet he/she calls task killers useless. Why do you think that rebooting makes their phone run better?
The O/S for the most part takes care of removing background apps when memory gets low *and* background apps *USUALLY* only take up a small percentage of cpu as a whole. However it doesn't explain why users' phones run better after a reboot. And, if you leave the app running the GPS in the background, well....
I just hate generalized comments to the new users that task killers are completely useless.