Here's what I tell my customers. Yes, I sell phones.
ATK and all related "Task killers" are like dropping a nuke on your operating system. It levels everything. It also destroys the little ecosystem of apps on your device. Most of the apps that pop up in the background are there because they're needed for something. Load a new song and all the media programs on your device stick their little heads up and go "Hey, do I need this? yes/no" Then go back to sleep. The fact that they are open in your memory doesn't mean they are running, consuming resources, or anything else. Android will clear memory as needed. It takes no more energy to run occupied memory than it does with unoccupied memory from what I understand. Task killers should only be used very sparingly, on active apps you are done with. There are no "task killers" built with this idea in mind. If you want the ability to see what's using your system resources use watchdog task manager lite. It will show you what is using resources, how much, and alert you if something is in run away mode.
Problems I have directly verified being caused by ATK and similar apps include, but are not limited to: Broken bluetooth, not getting mail, camera not working AT ALL, widgets failing to update, alarm clocks failing, calendar events not saving, or alerting, poor battery life, text messages not arriving or being sent, and various app specific failures and freak outs.
Juice Defender is a nice idea. It does disable certain things that will consume your batter, and has the potential to save you battery life without causing harm to your system for the most part. I have had people report things like email not showing up for LONG periods of time, and widget updates being broken.
I advise flashing a super clean rom, or debloating the one you have for best success with improving battery life. Beyond that, check your battery usage under settings and see if you have any culprits.