I know it's an old debate and I may not change anybodies mind here, but that's fine. A lot of people use task killers, but most don't use them properly. I came across this article today that explains quite well what they do and don't do.
Enjoy, and share your thoughts.
http://phandroid.com/2011/06/16/android-psa-stop-using-task-killer-apps-now/