I'm not a fan of this idea at all, for several reasons. Comcast might promote this as a value-add for their customers, but I really don't see how it can be.
First, there's the whole "shared spectrum" bit. Comcast was absolutely right to point this out, and really would have been negligent to not do so. What they don't really do though is tell you exactly how shared that spectrum is. While 5 GHz adoption is on the rise, most Wi-Fi these days still operates in the 2.4 GHz range. Of the 14 channels available in the 2.4 GHz range, only 11 are legal to operate on without a license in the U.S.. Within those 11, the most channels you can come up with that don't have any overlap (i.e.: where transmissions on one channel will not at all interfere with transmissions on another) is 3. Normally, this means you can have three people having their own Wi-Fi networks within close proximity to each other without having any toes stepped on. With Comcast's dual Wi-Fi setup though, it would be literally impossible to have two Comcast customers residing next to one another without having some amount of interference because each customer will now be having to use two channels in the 2.4 GHz spectrum - one for their personal Wi-Fi and one for the public Xfinity network.
Then, there's the issue of security, and this comes from two angles. First, there's the security of your home network as a Comcast subscriber who is hosting one of these hotspots. Do you really trust your ISP to set up a system like this properly and securely in one small device? I personally have a hard time trusting them to get the whole cable modem/router/switch/Wi-Fi combination right in the first place, without having to add in a second network. However, also consider the security risk they're putting their customers at when they're not at home. Comcast is telling all of its customers that it's safe to connect to any "xfinitywifi" network and provide their Comcast credentials to it to get online. It sounds to me like they're setting up their customers to be fraud victims more than they are helping them get free Internet access.
All in all, I'd say it's a bad idea for both Comcast and the consumers involved. Fortunately, I'm neither.