Alan, I value your expertise and opinions on the War, and I agree that the State Guards were not combat units per se, however, they were charged with LOCAL DEFENSE, be it capturing deserters, fighting roving bands of thugs that terrorized the the local home folks, And to help fight off all enemies threatening the local inhabitants. This would include, if they were needed, Sherman's advancing army. In the case of Cassville, Manassas, as it was named at the time, I doubt if the State Guard was reactivated officially, but I do think the local men in the unit were called on for help, maybe not to go to the front with a rifle, but to help Gen. Johnston's army in the many hundreds of ways that is needed to fight the enemy.
As to your question what prevented them from joining a regular combat unit, most of the men in the State Guards were much too old to fight in combat, boys that were too young, or, men that were physically unable to be in combat. An example was my gggrandfather who was 59 years old and one of his sons who was 15 years old. They both served in the State Guard

What really angers me , and it was not yours my friend, was the post comparing all State Guard units to the one portrayed in the movie Cold Mountain. Nothing could be farther from the truth !!
I know that there were some units as portrayed in Cold Mountain, but the vast majority of the State Guard units were patriotic men who wanted to help their country as best they could, and I feel it is a dishonor to the brave men who did the best they could to serve their country and for someone to say otherwise !!!