After all Curtis had an 11,000 man veteran army under his command, which was as large an army as Frederick Steele had in September '63. If Curtis had been able to take Little Rock the Confederates would have been forced to withdraw back to the Red River because of the Drought which was already in progress in May and June of '62. The Confederates were highly dependent on the food and corn supplies along the Arkansas River. This would have also meant that Curtis would have had to depend on foraging parties to maintain his hold on Little Rock. Those foraaging parties would have been highly exposed to the resistance of the confederate cavalry.
However, IF Curtis had taken Little Rock in May of '62, He could not have held it longer than his supplies lasted. The Union navy did not have control of the Mississippi River below Memphis until June 6th. This would have precluded any resupply by way of the Arkansas or White rivers until after the naval Battle of Memphis that destroyed the Confederate fleet. That was the significance of the Battle of St. Charles. It was the first attempt to resupply Curtis' army by a river route. The shot that crippled the USS Mount City, one of the Union river navys strongest city class ironclads, was in fact the shot that changed Curtis mind and saved Little Rock.