I leave mine on as well, and plugged into the charger. I do this for a few reasons:
1) Updates. Most of my apps will update over Wi-Fi during the night, so I don't have to worry about them. Llama automatically turns on Wi-Fi when I'm home so I don't worry about that.
2) I have my news feed set to download new items at 5:30am, so I have them to read on the train during my commute.
3) So I can check weather on the phone and know how to dress, without having to boot up a PC.
4) I have a reminder set to take some meds each morning, and without it, I'd definitely forget at least once in a while!
5) I work for the federal gov't, and during the winter, if there's a really bad storm, the gov't will shut down. There's an app that give you the current status right in the notification bar, so I can tell at a glance whether I can go back to bed.