That's just not right. There is not "rule" as to how to use a DVR - I may choose to save the entire season of shows to watch during the summer re-run months. If your theory were correct, why on earth would you need 100 hours of recording space?
Well it is right. Sorta. The weakest part of any DVR is the hard drive which is spinning at 7200 RPM constantly with heads that move back and forth across a metal disk covered with microscopic magnetic partcles.
Someday its gonna go south. Its all a question of when. Even if the electronics inside the 522 fail, when you send Dish the unit, they aren't going to transfer any programming to a new unit.
At least with DVD and tape, you can archive and hopefully save programming to something that is less likely to fail. DVD's get scratches and tape stretches and breaks. There is no 100% solution.
Rule of thumb: If you record it, make sure it is something that you don't care about losing at some point in the future. If you want long term storage, consider DVD/tape.
Eventually even 100 hours of storage will fill up and you will have to delete programming anyway.