I would say that he's not actually perfect for setting up the defense. Almost half of Jon's comments when he's in command of the Wall are "We (Bowen Marsh) should have built these things. Or done this thing." The trebuchets were set up poorly, there were many different pieces of counter-siege equipment that should have been built and weren't. He left Castle Black with a pitiful garrison that would not have been able to hold the Wall without Donal Noye's ability and Jon Snow's training and leadership. (the second factor he could not predict.) Ironically, Bowen Marsh's complaints are largely self-defeating. Jon/Stannis's policy of settling wildlings in the Gift is the answer to the problem of not having enough turnips. They also solve the problem of not having enough stewards and builders to repair the Wall, that's thousands of people able to cut back the woods and haul lumber and stone to build things.
You're right about the tradition thing. At some point people who can read need to sit down with a map and look at the Wall and the map of Beyond-the-Wall.
They need to ask themselves "If the Wall was meant to stop these wildlings why doesn't it go through the Frostfangs to the western shore? Why is the Gorge left unblocked? Why aren't there forts and watchtowers in the Frostfangs? Most importantly explain the Fist of the First Men, It's existence does not line up with the known history of the Age of Heroes and the Dawn Age.