DVR will receive updates whether on or off, it comes thru your cable line. DVR will turn on to record scheduled recordings, if your tv is plugged into the dvr your tv will come on also. DVRs do NOT defrag. They do not have defrag software loaded, the only thing even remotely close to this is a total wipe process.
Unplugging a dvr from the power will resolve the known issues like dvr reading 100% full when it is not, recordings playing back black screen, missing recordings. It will resolve losing all recordings only if it is a software issue, if your recordings are erased spontaniously that is a known issue and nothing can retrieve them.
Comcast agents may recommend always on because too many customers call in to say their cable is out when it is the box is powered off. (or wrong tv channel, wrong tv input, tv not even on, power is out, on and on and on...and omg what is that message light and why do I have messages?)
Why not leave it on? So many people cannot get the hit tv button and then power button to turn on tv, then cable power to turn on box. The ALL ON button on remote has its own issues, if you do not hold it down long enough you get all out of sync and goofy....
DVR...on or off your choice. My 3 are always on.