There are many issues that Comcast needs to answer before this should even be rolled out.
1) Does the "open" SSID have a separate IP address? If not, then what keeps someone from doing illegal things? Sure you need a username and password but if you have two "Guests" then which one was it?
2) Is there enough security in the RG that it will truly keep "Guests" off of the inside network?
a) Many companies that make residential products are horrible insecure. Arris cannot even get ARP right. Their handling of ARP is horrendous and worst yet, you can see ARP requests on the inside for the outside interface.
3) How do they distinguish between th traffic from one SSID from the other?
a) Couldn't users share their login information with a friend and they each use the others login information so that they use the "Guest" portion for their surfing and it doesn't hit their bandwidth cap.
b) Or does the RG report back with how much a particular user used and it deducts that from their allotment?
4) If someone buy X amount of speed, does the "Guest" deduct from it?
This just all seems like a very ill conceived plan from Cum-cock.