I do not believe that anyone can stay focused on a two hour stretch (assuming that breaks/lunches are issued every two hours) even when it's as simple as stopping people coming in the wrong way. When I used to execute Red Team attacks in the Army, one of our techniques was to learn the shift hours for perimeter guards (easily done through reconnaissance and observation), and wait towards the end of their shift before conducting our "attack" on the perimeter security (either surreptitious entry or a combat raid, depending on the mission and other considerations).

I consider the exit the most vulnerable of all checkpoint duties because that position is manned one-deep. The longer you keep someone in a static position, the more likely they are to lose focus as time elapses. And assigning light duty TSOs to the exit for a four or eight hour stretch (depending on their shift hours) only invites trouble. This is especially true if they're injured. Means that they're busy feeling sorry for themselves rather than concentrating on the job (yeah, unfair statement, but I think you know what I mean).

Yeah, I know. I can't understand why the other leads don't see it that way. I chalk that off to lazy fire-and-forget "supervision": put 'em in that position and only worry about relieving them for breaks and lunches. Pretty lazy. The checkpoint is no different than any other activity: noise means that things are happening; silence means that there's trouble somewhere. Oversimplified, but it's a lot like having kids in the house: if it gets quiet and stays quiet, better call 911.