First, there isn't enough technical info about how the feature is implemented for me to not be worried about it affecting me in terms of security and bandwidth; and from my subjective experience so far, I don't trust Comcast enough to accept their claim of no / minimal impact.

Second, the way I see it, Comcast is asking us to donate our resources (bandwidth, etc.), as minimal as they may be. Now, I don't mind donating to a good cause. I do, on the other hand, mind being forced to do so, without my consent; and especially so when a company is profiting from it ($3 daily access pass for non-subscribers).

So, if (a) I could be sure that it will not affect me, and (b) this would be a free for anyone who is in the area to use, and (c) they'd ask me to opt-in, I'd enable it.

It's a public wifi network that is broadcast from a device that is (probably) on your LAN. Attacker connects, finds some vulnerability, and has unrestricted access to the LAN. That is just one attack vector.

;tdlr it degrades your security, and is generally annoying.