So if I'm reading this correctly, you're saying with only the Genie plugged into ethernet, I'll get On Demand access through the HR24. No different than with the DECA setup. Is there a specific reason(s) the installers wouldn't do it that way?
Any good installer is going to want to customer proof your system. They are going to install a BB deca right beside your HR34 and plug your internet into the BB deca.
Why, because when you decide that you want the other room to be able to record 5 shows, and only need 2 shows in this room, and you call *D to ask them if you can move the IRD, and the numbnutz on the phone says, "sure, no problem, it will work just fine." and you do. Your system no longer has internet connected to it, and your HR24 stops sharing with the network. Fast forward 24 hours and your next call to *D results in a truck roll and a chargeback to the original tech.
** Not to mention not installing the system this way negatively impacts his pay for your job, plus it negatively impacts his performance metrics that his entire paycheck is based on.