We just completed my kiddo's 19th flight (I didn't go on a plane until i was 15; he is only 4!) and I have never flown with a car seat in the cabin of the plane. With the exception of one trip, we have always brought one and gate checked it. We use the GoGo Babyz attachment to turn it into a stroller/wheeled luggage cart and use that to easily push it around the airports. The whole thing gets checked at the gate for free.
I have also never used the CARES harness so I'm not sure how that works.
As someone said, it is a lot different to be on a plane than in a car. In fact, we always ride trains, planes, subways, busses, taxis, etc. without car seats. Pretty much any public transportation. However, do what works for your peace of mind and safety.