I would really like the idea of more than on per account, however, I doubt we;ll see that ... they wouldn't allow more than one of the older Genies on an account ...
Now as you mentioned, the Smart thing to do would be to allow all the HRxx boxes to work with the HS17.
The Genie 2 uses a different method to associate clients with the server, that avoids the problems of Genies "fighting" over clients that were experienced by those (employees, CE testers) who had more than one of the HRx4 Genies on their account. Maybe they had other reasons for making that change, but one of the outcomes is that it avoids the need for software changes to handle multiple Genies if they are all Genie 2.
Even if they hadn't made the decision to not allow older hardware on an account with a Genie 2, adding a bunch of H2x/HR2x to service larger accounts is simply not feasible since that hardware is getting older and will become obsolete when real 4K channels finally begin to appear.
Some people think Directv will simply abandon those higher end customers since they're a small portion of all Directv users, but that doesn't make sense since they've pursued a strategy for years of going after high end customers. That's why their average customer spends $128/month, which is quite a bit higher than any other cable/satellite providers. That number would drop a lot if they basically told every customer who had more than 4 or 5 TVs, or more than one 4K TV, to take a hike which they'd effectively be doing if they limited everyone to 7 tuners or made them choose an HR54 limited to a single 4K TV and pay to lease obsolete HR24s. People are willing to do that for now because there isn't much 4K to watch and those old HR24s are still working OK, but if that's still the situation at the end of next year those people will begin looking elsewhere unless they're locked in by NFLST.