Under normal circumstances, I agree. However when an app starts misbehaving and eating up the CPU, it's nice to have the option to kill the app yourself. Also there are times when an app is locked up and killing it is the only way to have it start fresh.
And the Google Android team must agree because they provided an easy way to kill an app (swiping the app right/left in the multitasking switcher). But in addition to that, I still use an app called Watchdog to monitor CPU utilization. It notifies me if an app exceeds 40% CPU utilization. So if I have an app I was previously using pop up with high utilziation, I'll kill it. Why wait for Android to kill it eventually when I can do it now and save battery life?
But other than those types of scenarios, I let Android handle task management.