either the PT is a moron or he is just saying you should balance out your muscles when you lift.
it is true if you just go in there like a baboon and do nothing but flat bench every workout for the next 10 years you will develop tremendous imbalances, but his reasoning explaining that to a newb it sounds like is idiotic.
I do think guys should attack their weaknesses more in the gym. I see guys only work their strengths and ignore their weaknesses way too much.. obvious example are juice monkey's who have huge upper bodies and twig legs. less so the guys who walk around with their chests puffed out but can't even pull 50 pounds.. you can bench 400 but you can't even pull or squat half that.. pathetic!