I also was confused by the Dragon strategy. It would seem like she should have made a second or third pass up front to completely destroy the defenses. I know the more she exposes the Dragon the more danger it is in but the best time to attack was while you have the element of surprise before they can organize any defense against the dragon. Seemed to be more of a cinematic choice by the show runners that it made for better TV if you let the horde kill a few.
A. She might not have total control of him, or it might take time to get him positioned exactly where she wants.
B. I was expecting them to approach the Lannister battle line from one side (parallel) and torch a long swathe of soldiers, rather than doing what they did and hitting the battle line on a perpendicular approach.