It's not clear what the "nightly" thing is anymore. Years ago, with other receivers, the nightly reboot was definitely related to guide updates. With the Hopper, it's been stated by many different people, including Scott, DIRT people, etc, that guide updates can and do happen at anytime, 24 hours a day, as long as a tuner is available. What I, and others, think the nightly reboot also is for is simply stability of the DVR. It is just a computer, after all, and reboots always help. When we had the 522 receiver, I made a habit of rebooting it every few days and it worked real well.
Some think that it has to reboot just to check for s/w updates, but that seems rather strange (from the computer aspect of things). Then again, maybe it only checks during an early stage of the boot-up, before the normal OS loads ?