... but their Xfinity Wifi Hotspot program, if implemented correctly, shouldn't cause customers any real harm.
What I believe happens is that your modem gets virtualized into two modems/routers. Cable Internet is already based on shared broadcast signals, so in terms of bandwidth it should be identical to adding a second, mostly inactive cable modem somewhere in your neighborhood. Since the 2nd modem is virtualized, it should not affect your transfer rates or bandwidth quotas.
This second modem is connected to a second, virtual router, with its own SSID. Unless there's a vulnerability in the router (which is possible), users of the Xfinity Wifi Hotspot should not be able to access your network, use your IP address, etc.
Available bandwidth could conceivably be reduced, due to more packets in the air, but WiFi is already unregulated and subject to additional interference. Increased load on the modem/router could theoretically reduce your bandwidth as well, although probably not by any noticeable amount.
The best claim is based on increased electricity usage. However, the additional energy needed is probably negligible. Here is a link to a blog post about the increased electricity costs, where they conclude it's about $8 per year in the mid-Atlantic area -- if it's being used. Comcast could give everyone a $1/mo credit for enabling the Xfinity WiFi Hotspot, completely eliminating the issue.