When using NAT, you're revealing the IP address of your router. I don't know about you, but I don't have so many devices running on my home network that would drown out what I'm doing.

With NAT, you can still receive DoS attacks, still have your game networking exploited, and still be geolocated. The only remotely security-related benefit is that instead of your ports being exposed to the wild internet, they're exposed to your router which is more of a side-effect rather than an actual benefit. Its not a reason to not bother having a firewall.