Yes, the author presents the range of theories without obviously favoring one view over another. The sticking in my own mind comes from the perspective of loyalty and morale. I would think the last group an emperor would want to pay with "funny money" would be the army.
Great thread! Whether a form of military scrip or simply made out of necessity, I have always felt that limes denarii were almost criminally under-valued on the marketplace. My favorite are this set of Marcus Aurelius and Faustina Not sure about the Faustina, but the Aurelius dates to the start of the Marcomannic Wars, and I like to pretend that it may have been used by a soldier fighting on the frontlines of that war. I also have a Severus limes, seems to be solid but more than a gram too light, so I assume it's billon cut with copper and other lighter elements? There's also this Caracalla, clearly not an official piece because it has the obverse legend BRIT dating it to late joint reign / early sole reign, but the reverse is a type used only by Plautilla, who was already in exile when the British campaign was underway.
Szaivert (MIR 18, pp. 230-231) dates the SALVS issues of Faustina II to a period of uncertain length beginning in AD 165. He notes that the end of this phase is currently uncertain, but suggests a possible break in the issuing of coins for Faustina in the late 160s. The fact that no reverses struck for Faustina which appear to celebrate the elevation of her sons, Commodus and Annius Verus, to the rank of Caesar in 166, or the birth of a last daughter in 169, suggests that the issuing of coins for Faustina did not resume until possibly 170, when coins depicting her in her final hairstyle appear. So your Faustina coin was issued right around the start of the Marcomannic wars.
There are so many of these "limes" coins that they must have been actual coinages and not contemporary counterfeits. Oberlander-Tarnoveanu considers them as products of the military units operating on the Danube starting with the 3rd century -- so likely the coins with earlier 2nd century effigies were actually minted long after the emperor depicted on them was dead. As a necessity coinage, the minters had to also take into account the law against counterfeiting the reigning emperor's coinage and this probably accounts for the copying and mule-ing of earlier coinages.
What is very interesting in my opinion is the copying of scarce or rare issues or the downright invention of issues that are not (yet) known from the original and official mints -- the double effigy of Caracalla and Geta from @DonnaML the CASTOR someone else posted, the "quinarius" of Alexander, @Finn235 's Caracalla-Plautilla mule, there was even a "antoninianus" of Alexander in 2015, see here. It also seems to be some tradition or underlying conditioning that evades us: in the first half of the 4th century we see this again, but this time with struck semi-official or semi-barbarous coinages of very rare types or invented types that were not coined by the imperial mints. This happens this time on the Rhine and in Gallia proper. See for instance Constantine II holding spear and globe on a BEATA reverse. Almost good enough to pass as an official London mint. But not quite there. And completely unknown as a type from London also: Or Licinius II with consular legend from Lugdunum? Unheard of. Where these innovative types purposefully made like this as to not be seen as counterfeiting the official imperial mint coinage of the day?
@seth77 , I forgot to post my unusual mule, with an obverse of Julia Domna and a reverse of Plautilla. Considering that Domna was Caracalla's mom, and Plautilla was his wife, i suppose it's possible that this die combo was made officially, but there is no evidence of any.