I suspect that it won't be long until keeping it off your home network is meaningless.
Many smart devices already search for and connect to open networks. Not an issue for most tech-savvy people who maintain their own networks, but there are a lot of people out there that either don't know or don't care that their telecom-provided gateways maintain an open wireless network by default, in addition to their private networks for home use.
My pessimistic take: the intent behind this is to create a de-facto mesh for IoT devices in areas where people live close to each other, i.e. apartment buildings and townhomes.
Integrated cellular modems are relatively inexpensive; it's not too far of a reach to think that soon, IoT devices won't need to rely on wifi to send their surveillance reports targeted marketing data.
