Why don't they just use the existing network?
It would give their support managers nightmares. They probably don't want to support thousands of different potential network configurations. Do you have a real cat5 cable or a phone cable? Are there any mac filters on the router? Are you plugged into a LAN or WAN port? Etc, etc, etc. If it truly is restricted to MoCA only, then they have a reasonably homogenous system to support.
I would hope in the end they open it enough that you have the option of using standard ethernet at your own risk. With both the 813 and 110s having connections, I think it will probably happen (but have absolutely no basis for that belief).