They get clogged over time. It is incorrect for that exhaust guy to say they will go bad from less back pressure or any other ridiculous statement like that. They simply get clogged up over time from the exhaust gasses (the inside of a catalytic converter is a honeycomb like material). Usually a catalytic converter will last a minimum of 100,000 to 200,000 miles.
Breathing in and out is good enough for me.