If Adam was posted as guard, then he was supposed to resist. Even in an unfallen world, Adam had to resist evil; my argument in Defending Constantine is that the fall was, among many other things, Adam’s failure to resist. If Witherington denies that Adam was supposed to guard, it follows that he was not called to resist. Perhaps that’s the real brunt of his argument. If so, he’s correct: If Adam is a guard, he’s guarding against something, and that means there’s something for Adam to be against even before sin enters the world.
In responding to Ben Witherington's critique, Leithart enlists eminent Biblical scholars to his defense. I would add one more, Gregory Beale, who spends some 20 pages in The Temple and the Church's Mission defending the link between the garden and the temple and Adam and the priests, and who endorses the notion that one of Adam's responsibilities was in fact to guard the temple. As he puts it:
It is apparent that the priestly obligations in Israel's later temple included the duty of 'guarding' unclean things from entering (cf. Num. 3:6-7, 32, 38; 18:1-7), and this appears to be relevant for Adam, especially in view of the unclean creature lurking on the perimeter of the Garden and who then enters. Interestingly, priests of ancient pagan temples were also to 'guard the temple' and to kill intruders, as well as to 'guard' and pass on sacred texts.
That much Leithart says in his post. But Beale goes on to point out that when the Fall occurs, God "stationed the cherubim...to guard the way to the tree of life" (Genesis 3:24). Such is the difficult irony of Christianity--the one who is meant to guard is now guarded against, kept out by the same sort of creature which Satan had once been.
What implications this has for political theology are, of course, not obvious to me, especially given the division between the priesthood and the kinghsip later in the Old Testament. But the point about 'guarding' the Garden seems to be, as an exegetical one, reasonably sound.