Wouldn't putting the device in airplane mode do the same thing as JD then? Why need an app?
I use Green Power, which is similar to Juice Defender.
What they do, as far as I understand, is turn off wi fi and 3G/4G when the screen turns off, but it (Green Power and, I believe JD) will turn them on and sync for one minute, every 15 minutes (or you can configure it for more or less frequently)
That way, while your phone isn't syncing all the time when the screen is off, it still does check for email, etc. while the screen is turned off.
Also, I believe with Airplane Mode, you wouldn't receive phone calls, as airplane mode turns off ALL radios. With GP and JD, you'll still receive all of your phone calls, and all SMS messages. You won't get emails the minute they arrive, but you will get them within 14 minutes or less.
They also check to see if, for example, you're currently downloading something when the screen turns off, and they won't disconnect you until the download is completed... again, something you wouldn't have by simply turning on Airplane mode.
It's a VERY elegant compromise, unless you absolutely need to know as soon as possible when any email arrives. I've found my battery life with Green Power (and, previously, with Juice Defender) to be much longer then without them.
Your mileage may vary....