It was the key to the downfall of the Roman Empire.
It was key to the rise of the empire. Especially post Caesar. The first two centuries AD were the most dynamic period of the empire. Without the integration of the "barbarians" in the outer provinces, e.g. Gaul, empire would have been impossible. IMO.
I would agree that empire has it's costs. It is probably not sustainable in the long run. Due in large part to what you asserted.
Contrary to what many believe, the Roman Empire saw it's best days after they shed the chaotic and corrupt Republic and embraced the reign of the emperors.