Um, there is no conclusive evidence that says "you shouldn't use task killers."
I use one, and it's great when used sparingly and in certain situations. Some apps do not completely close, even if you tap the back button multiple times to back out. Apps include Last.fm and Pandora, MLB AT- Bat, and any other app that streams a large amount of data to your phone, either audio, video, or both.
ATK comes in handy since you can end them completely. Failure to do so may sometimes lead them to continue using data connection, even if the app isn't open and killing your battery just as if you were using the connection.
This may only be an issue with the Eris, as buggy of a phone as it is, but it is definitely good for when the phone starts to slow down in general.
Restarting the phone doesn't take a long time and it clears up RAM, but I don't make it part of my daily routine to restart my phone. My phone is currently on 420 hours of runtime and counting since its last restart. It's running as smooth as it has ever been.
So use ATK for apps like those and for times when your phone really starts to become sluggish in the menus. Yes, the OS cleans up garbage files and may put some programs in the background, but it isn't so advanced that it can conquer this task on its own. It's a nasty rumor that started somewhere and has spread like wildfire since.
At the end of the day, you may or may not need to use ATK, depending on how heavy of a user you are. But a widget can be placed where all you do is tap the robot icon (the widget) and it will kill all tasks on the spot. No more checking programs one-by-one to end them. One click, kill everything. Works fine for me.