"The Brits had a similar system, and for the same reasons the French did, which was to protect their colonies. While there are several advantages to having a multi-cultural force composed of U.S. citizens I don't see any advanatages to recruiting non-U.S. citizens to do our bidding."
Oh, I don't know. The British Army found Ghurka units to be exceptionally reliable long after the Sun set on the empire. The French still maintain their Legion for the same reason. Psychologically speaking, there is and always will be, a role for the employment of "scary" shock troops who are different from the rank and file.

When a republic chooses to have a professional, rather than a conscripted, military, high quality auxillaries have their place, in the appropriate ratio.