You may not use three tuners at a time to watch, but I know I've had all four of my DVR tuners (2xHR24) recording while watching something on another box wishing I had more recording tuners. I wind up with a lot of conflicts on Sunday and Monday nights (and it will get worse very soon because all the new seasons are about to start).. With the HR34 (Which is being rebranded as the Genie - hence "Genie Approved"), if you have four recordings going at once and are watching a fifth program on the local TV, then the C31 clients are SOL. They are stuck watching pre-recorded content or a recording-in-progress because there are no available live tuners.
I am looking to possibly get an HR34 once my contract is up to replace one of my H24's just because I need more recording tuners. If there were no monthly charge, I would absolutely get a C31 for the guest room and office because the guest room isn't used that often and I'm the only one in the office so if that's on it means I'm not in one of the other rooms.. But you are absolutely right, given my DVR habits, I would not give up the extra tuner and still have to pay the full mirroring charge. If they have to charge a fee on that, it should only be a buck or two tops. Not only that but isn't the HR24 already a higher fee anyway?