Personally I think Johnston was a horrible general....He may have been a great tactician, but was an awful strategist, which was exacerbated by being insubordinate.
At Vicksburg the union goal was simple, to open the Mississippi to Union control and split the confederacy in two, to which Vicksburg was the key due to its collection of batteries and position. If Johnston had abandoned Vicksburg for 2 weeks and then by maneuver checked and pushed back Grant, The Union would have still destroyed the heavy guns at Vicksburg, which the south at this point could ill replace, and would have still accomplished its goals.
The same is true later at Atlanta, both sides (except apparently Johnston) realized a victory for the North could have a huge impact on the presidential election, also at this point the south could ill afford to lose one if its few major transportation , supply and manufacturing centers. So once again if by "saving" his army to fight another day, even if he checks Sherman later, he failed again, And at this point I use "saving" his Army, because morale is declining due to constant retreats and he's losing his army to desertion.
His inclination to look at things more tactically then strategically wouldn't have been a major problem if he at least followed the orders he got, because they in effect would have defined the strategic goals for him. But that's what is most frustrating reading about Johnston's campaigns......He knew he was supposed to hold Vicksburg, and go to it, Pemberton could not move to him without abandoning all the heavy guns that gave Vicksburg its importance. He knew he was supposed to make a stand to defend before Atlanta. Yet would simply just ignore his orders.