The security of sharing your service with others is questionable, for starters, and Cox does not have a good security history. Every "reset" of the modem (where they remotely check in on it) will turn off your preferences, and many people have reported disabling this wifi "sharing" only to come back to see it re-enabled.
Cox, or any ISP, having access to your internal network by virtue of having administrative access to your modem / NAT router is problematic. People can be bought or are sometimes evil. People click on links and get compromised. It's better that they stay outside of your home network, where they belong.
Cox rents this modem / router device for $12 a month. That's $144 a year. A high end DOCSIS 3.1 modem would pay for itself in less than a year, plus you can get a NAT router / wifi access point of your choosing, keeping the function of the two devices separate. This means that if you want to upgrade wifi later, you're not paying to replace everything. Or if there's a security issue that the vendor isn't going to fix, or is in hardware, like the Intel Puma chipsets, you're only replacing one of the devices.
This is true of all ISPs, really.