It's not that I "believe" I get better battery life with Automatic Task Killer, it's that I actually do. I've posted a thread on here where I went about a week without it and a week with it. The only difference was the task killer. The week with it was significantly better (based upon similar use) than without it. I even repeated the process again to see if the results would be the same and they were.
The bottom line was without ATK, my battery would be dead by the time I got home. With ATK, I would go to bed and still have more than 50% left.
I still think that Automatic Task Killer gets confused with the other one and that if people actually paid attention to what they were using and used only Automatic Task Killer, they would find a significant increase in battery life.
Also, I think that there are certain problematic programs that require a task killer to shut down or will drain the battery signifcantly. If you don't have one of those programs installed, then you wouldn't benefit from it.