A couple of things she says need interrogating. She points out that the common picture, of Hadrian's Wall as an extended city wall from which barbarians were repulsed, is incorrect - the Wall was never sturdy enough to repulse that sort of military assault. That's all perfectly true, and in any case the Romans didn't conceive of defence in that fashion - the Roman army defended the provinces by drawing attacking forces into open combat and wiping them out. But such an observation applies only to the actual curtain wall and associated ditches. The whole complex of Hadrian's Wall includes forts where the troops were based, milecastles, turrets for observing activity in the frontier zones, and outpost forts beyond the wall. It's a lot harder to argue that these could have served no purpose in the military defence of Britain.
And I wonder about the way she characterizes the way the Romans thought of the frontier. She says:
The Roman image of the frontier was usually much more subtly nuanced. The empire shaded into “foreign” territory across many kilometres that were melting pot of cultural difference and often a hot-spot of trading and commercial activity. It was a question of frontier zones, rather than frontiers – governed partly by Rome, partly by a whole variety of non-Roman powers.
Now, it's certainly true that the Romans tended to think of external kings who had made agreements with Rome as being essentially part of the Roman empire - 'client kings' were as much as anything an alternate method of governing areas within the empire to direct rule from Rome. Moderns don't always recognize this (I encountered this recently on a guided tour of Colchester Castle, where we were told that Prasutagus of the Iceni was not amongst the eleven unnamed British kings mentioned on an inscription in Rome as having submitted to Claudius, because he remained king - I'd argue that retention of his crown on Roman terms was, to a Roman, submission). But the picture Beard presents looks like a modern archaeological way of looking at the frontiers, rather than necessarily something a Roman might have thought.
I don't know of any ancient sources that present quite the view Beard has, though I'm happy to be proved wrong if anyone can cite something. There are plenty of texts that talk as if Rome has no limits at all, and the world is divided merely into areas Rome rules and areas Rome will rule. But, if I recall correctly (and again, please correct me if I'm wrong), many of these date to the Augustan period. There's a danger of being too simplistic when one talks about 'what the Romans thought', and forgetting that over a period of several centuries, prevalent opinions could change.
Hadrian's Wall is not quite as unique as Beard implies - there are other frontiers in Germany (where the linear barrier is a palisade), in North Africa (where the forts are scatted along a road), and of course the Antonine Wall, further north than Hadrian's Wall in Scotland. It seems to me that it was becoming possible in the second century for some Romans to think in terms of linear limits to the Roman empire, even if they still understood that their influence could project beyond it (which is no different from modern nations who believe that their influence goes beyond their borders).
What cannot be questioned is that linear barriers were built. And once one builds a fence or a wall across a piece of ground, one is making statements about the territory either side of that barrier. A barrier like Hadrian's Wall carries with it an emphatic statement that the territory behind it was Roman; but it also carries a (perhaps less emphatic) statement that the territory beyond the wall, if not entirely given up by Rome, was certainly somewhere that Rome did not maintain such a strong claim to.
Let's return then to George Bush's fence. I think this is rather more like Hadrian's Wall than Beard allows. Many purposes have been suggested for Hadrian's Wall, from giving the soldiers something to do, to securing lines of communications. beard lists most of the suggestions. It is entirely possible that many of them are valid - indeed, I think it likely; I've always been suspicious of attempts to find the One and Only Reason for historical events, as single causes rarely tell the whole story.
One of the suggestions is that the wall was meant to control the non-military movement across the frontier zone. I think there's a lot to be said for that, and if that's true, then is that not exactly the same as the stated purpose of Bush's fence? Bush wants the fence to prevent illegal immigrants crossing from Mexico into the US. This is what has been suggested for Hadrian's Wall - making sure people could only cross the frontier at manned posts, where they could be taxed, and their right to enter the empire be confirmed.
It has also been suggested (and again I'm sympathetic) that, regardless of any practical purpose, Hadrian's Wall had a symbolic function. It was meant to show to anyone crossing the frontier that they were entering a more powerful state than they had left. The Bush fence seems to me to fulfill the same purpose (as do the hoops that Immigration makes anyone trying legitimately to enter the US jump through).
And propaganda goes both ways. There's a case for saying that the Bush fence is meant to assuage the fears of people living in the southern States that they are about to be swamped by immigrants. Something Must Be Done, and it doesn't matter if it's practical, or if the threat is genuine, as long as action can be seen to have been taken, and that action can be seen as resolute.
And so I find myself wondering whether, if we knew more about the second century AD in Britain than the anonymous fourth-century concocter of the Historia Augusta did, would we see an emperor under pressure to Do Something about the barbarian threat, and choosing to produce something that might not keep the barbarians at bay, but let his subjects sleep at night?