If the militia is necessary for a free state, then the militia has to be affiliated with the state in some manner. Otherwise, it is simply a shooting club at best and a goon squad at worst.
Being as this country has managed to get by without any "organized militia" since the Civil War, I think it is safe to say that it is NOT necessary. If the state has continued to be free, it has not been because of any militia.
This may have been true in 1780. It seems to have been true when the War of 1812 started. Since then, no, not so much. The Civil War was not a bona fide struggle to keep the country free: the South wanted the freedom to keep many people in slavery, and all under the control of a plantation elite. Some of those in the North wanted to free slaves in the South, but the leaders in the North wanted to retain economic control of an important source of raw materials from the South.