Most of us on CHF think Dubs' theory is half-baked, and is only interesting to Western historians because they are obsessed with any reference to early contact between Europe and China.
I wouldn't go so far as to say "half baked" on the part of Dubs, but I think many of his supporters are (see David Harris). Dubs was an intelligent man, he did an important translation of the Han Shu as well as some important work on Confucianism.
The theory is untenable though. Here are a few points:
1. The Romans defeated and captured at Carrhae ceased to be an effective fighting force. Plutarch mentions in gruesome detail how those who were not killed were maimed horribly by the barbed Parthian arrows. Dio describes the same thing. In addition, the climate was so harsh on the Romans that many simply died of exposure. Romans and Gauls don't survive very long in waterless deserts.
The testudo failed miserably at Carrhae. It might have worked well against European barbarians, but not against Parthian bows. Chinese crossbow bolts would have cut through a Roman testudo like a hot knife through butter. Any Roman survivors of Carrhae would have likely abandoned this formation, for its effectiveness was severely lessened by the realities of combat in plains and steppe. Plus, it wasn't until Antony's time that the testudo was perfected, by presenting a complete wall of shields against missile attacks.
2. The Roman captives were not sent to Margiane to "guard" the frontier against barbarian raids. Dubs distorts a passage in Pliny which reads: "This [Margiane] is the place to which the Roman prisoners taken in the disaster of Crassus were brought by Orodes" (Historia Naturalis
VI 47). Pliny does not say anything about them "guarding" the border. Their position as prisoners
is stressed even here. Besides, Margiane is surrounded on all sides by desert, a natural prison.
Roman shields and arms were seized by the Parthians as trophies. Plutarch mentions a triumphal parade in which the spoils of the Roman force were paraded through the streets of Seleucia. In addition to this, Roman banners and icons were taken to Parthian temples as tribute to their gods.
Plutarch also mentions that some Romans were executed after the battle. A passage mentions "Roman heads newly cut off" affixed to axes of the captured fasces during the Parthian triumphal parade. It was a 1,500 mile march across desert from Carrhae to Margiane. How many died from exposure or execution? How many were sold as slaves? The remnant that reached Margiane was a pitiful remnant of POWs, not a force of border guards. It's absurd to assume that the Parthians would utilize Romans as auxiliaries while they were at war
3. From Margiane to Chih-chih's city on the Dulai river it is roughly 500 miles of the harshest territory on earth, the Kara kum desert. Temperatures there rise to as high as 172 degrees F. and the water of the Oxus River rises to 87 degrees. Human life during the day is almost totally dormant. Alexander the Great had trouble crossing this region, and he commanded a well-provisioned force!
That's just a few points on the western end of things.