[A.J.Armitage]

One could argue that Eastern Rome not had Persia as a buffer, it would not have survived the Islamic conquests.

Maybe. If they hadn't had all those wars, they both probably could have made it.

[Amaxen]

True, I don't know enough about the reasons behind why they occasionally clashed. I think Persia had other enemies to worry about and mistakes to make, as did Byzantium. In Byzantium's case, what brought them low was their attempt to reconquer the Western Empire.

Still, the Byzantine Cataphracts were probably the best heavy cavalry in world history, except for the Persian Dhegans, whom they were a pale imitation of.