I have just recently updated my Android Battery Optimization Guide to add Juice Defender and get rid off recommending Advanced Task Killer. The reason behind the switch is simple. I had no recollection that Juice Defender even existed, and through the Android Forums, I have learned that Advanced Task Killer is actually detrimental to your phone's ease of access, reliability, and approximated life.
The reason you do not want a task killer is simple. Your phones are designed to shut off apps when the app is done servicing you. If it is still doing something (roaming for network, syncing, playing in the background) and you shut it off, when you go to turn back on the app that you force closed, the app has to finish what it was doing, then restart all together.
If your apps are force closing often and you're using a task killer, the task killer is the issue. In the time where the app is resuming where it left off and trying to reset, if the developer didn't code it specifically for that instance, odds are it will have no idea what to do and crash. To prevent this, do not use it. It's that simple.
Now onto reason why you need Juice Defender. As I stated in Android Battery Optimization Guide, Juice Defender limits (but not destroys) the data usage allowed per app, depending on the settings you set. You can, however, permanently restrict the usage of network data. I do not recommend this, due to the need for network on your apps (Which is why they use it in the first place).
I just started using it, so I have not untapped the full potential as of right now, but in the past week I have been using it, my battery life has increased by about 10-15%. Just from the app. What I am basing this on is when I go to bed, I check my battery life (yes, my phone lasts the entire day, huzzah). Before Juice Defender, I had from 8-16%, now I have anywhere from 27-39%. Quite the improvement if I do say so myself, and it doesn't destroy, compromise, or mess with your phone.
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