A Well Regulated Militia

What is the importance of the Second Amendment's reference to a well regulated militia?  Josiah Quincy wrote in 1774:

“No free government was ever founded or preserved it’s liberty, without uniting the characters of citizen and soldier in those destined for defence of the state. The sword should never be in the hands of any, but those who have an interest in the safety of the community, who fight for their religion and their offspring; and repel invaders that they may return to their private affairs and the enjoyment of freedom and good order. Such are a well regulated militia composed of the freeholders, citizens, and husbandman, who take up arms to preserve their prosperity as individuals, and their rights as freemen. Such is the policy of a truly wise nation, and such was the wisdom of the antient Britons. The primitive constitution of a state in a few centuries falls into decay: –errors and corruptions creep gradually into the administration of government--’til posterity forget or disregard the institutions of their remote ancestors. In antient time, the militia of England was raised, officered, and conducted by common consent. It’s militia was the ornament of the realm in peace, and for ages continued the only and sure defence in war. Was the King himself general of the army—it was by the consent of his people.”