Sure, they aren't necessary to close a ton of programs, which will just slow down the phone, but I feel they are very necessary for battery life. Example:
I decided to stop using ATK for one day, today. At 1pm my battery was at 50% which is usually what it's around when I use ATK. 30 minutes later (1:30), it was at 10%. I couldn't figure out what was going on because I didn't use it AT ALL in those 30 minutes. The last thing I did was check my facebook.
Well, I found out that the facebook app was constantly trying to refresh my notifications, even after I closed the app. For some reason it got stuck on refreshing and wouldn't stop. Then I remembered why I use ATK. Because it stops crap like this.
I recommend using a task killer for apps like facebook and the browser. I usually kill both of these after I'm done using them. I don't kill any other apps that always reopen, like alarm clock, calendar, etc. I strictly use the task killer to kill apps like facebook, browser, games, etc. after I'm done using them. In my case today, using the task killer would have saved the battery.