It almost strikes me as a classical Vietnam in a way.
Why were two huge superpowers so focused on a relatively tiny scrap of land for so long?
Rome didn't have a precedent of fighting proxy wars. They attacked, did whatever it took to crush their rivals entirely and moved in. We see this in Carthage, Pontus, Macedonia, Gaul, and so on.
However, after Antony they largely confined themselves for wrestling over indirect control of Armenia.
Were there any practical benefits to controlling it that justified all the effort? Why wasn't there more conflict in other theaters?
This question has been answered before, here's the link if you interested:
http://www.reddit.com/r/AskHistorians/comments/1jx70t/why_did_the_roman_republicempire_and_the/
To expand a bit: one thing to make clear is that Armenia in antiquity was far bigger than it is today. Armenia was a buffer state between the two empires after the fall of Tigranes II, and having hegemony over it as your client state was imperative to having the strategic upper hand. From the Romans and the Parthians, to the Byzantines and the Sassanids, controlling Armenia was crucial.
For example, the Armenian king Artavasdes II of Armenia, who was an ally of Rome at the time, offered the Romans reinforcements in their invasion of Parthia in 53 BC in return for Rome invading Parthia through Armenia. Crassus refused the offer, and the Parthian army (which consisted entirely of horse archers) outmaneuvered the Roman infantry (despite their vast numerical superiority) in the flat deserts of Upper Mesopotamia.
This rejection of Crassus to fight in Armenia (a mountainous country which would have limited the maneuverablity of Surena's mounted archers) and to refuse the 16,000 cavalry and 30,000 infantry Armenia was offering, resulted in one of the most important defeats in Roman history, causing death of Crassus and the end of the First Triumvirate, and resulting in the onset of Ceasar's civil war and the formation of the Roman Empire.
In effect, whoever controlled Armenia had a huge strategic, logistical and military advantage. Annexing Armenia was nigh impossible at this time because of its vast size which overstretched armies, unforgiving terrain and its formidable army. So instead, both empires (and their successors) persistently aimed to secure Armenia's allegiance. This prestigious task was of utmost strategic importance, which is why the two empires focused so much on Armenia.
