I don't entirely buy it, although I study the late Republic, not the military of the Principate, and I haven't actually read the book. But there are certain problems I see. Goodman is not a historian of the political or military history of Rome, he studied Judaea under Roman control. While I certainly see this as being possibly valid in terms of Judaea, I noticed several inaccuracies here for everywhere else that might be important. The claim that armies in the Republic were much smaller is blatantly untrue. Octavian ended the War of Actium with over 60 legions, and some of Antony's legions still roamed free and had not surrendered. At Philippi and Pharsalus the number of Roman soldiers rivaled that amount, and each side fielded a number of legions easily rivaling the Augustan 20-some-odd legions. Even if we only look at provincial armies it's clear that this isn't really true. Each proconsular or propraetorian governor was allotted his own army, and though some governors going to easy provinces had fairly small armies others in provinces like Macedonia or Syria that were regularly threatened had much larger ones and could recruit more legions as they saw fit. Although these armies are essentially privately held, to say that they are not part of the "Roman army" isn't true--the claim made by Goodman only holds up if we mean consular or praetorian armies, and totally ignore the provincial ones, or pretend that they are totally independent. I'm also somewhat skeptical of the claim that Roman defense was all about retaliation. The Augustan army underwent a significant change, wherein legions were stationed along the frontiers, rather than kept in the middle of provinces when not in campaign, in permanent fortifications to guard against encroachment. The Rhine legions are the perfect example of this. Defense based on retaliation tended in the Republic to occur when things went wrong (like with Jugurtha) or when Rome was attacked and didn't expect it. I expect that such a model holds true for Judaea, but it's certainly not entirely accurate on the Rhine or in Macedonia. All this said, certainly politics had something to do with it. The massive down scaling of the army is usually assumed to be an attempt to limit the ability of individuals to amass huge armies capable of challenging the state (hell, the Res Gestae practically says as much) and the station of legions in Egypt has long been held as one of a number of reforms to prevent Egypt, a very valuable province, from bring taken over by an opportunistic governor. Egypt isn't a good example, as it was an exception to many rules--under Augustus it was governed by an Imperially-appointed equestrian prefect, while still having a full legion, because legally Egypt was the private province of the emperor himself. Africa is another example of an oddity that is relevant--despite being a senatorial province it held a full legion, to protect against tribal raiders who had attacked the province since it's existence. Certainly politics played a role in the restructuring of the army, nobody disagrees. But was it like this, an attempt to maintain political and social control over the provinces, or to prevent political rivals from challenging the state?

I don't intend to judge the book at all, since I haven't read it. Keep that in mind. I offer my opinion of certain points mentioned, and with luck someone who knows the work better will pick up