Musk has stated that they added the delta wing to the BFS design because; from one flight to the next the ship may have differing payloads and differing fuel loads. These changes shift the center of gravity of the overall ship. So the wings were added to compensate for this.
It seems to me that this is only a problem as the ship renters in a long side orientation like the shuttle, rather than an engine first orientation like the Falcon 9. An engine first orientation would probably not have this center of gravity problem, and need less heat shield.
Does anyone know why they have chosen to reinter this way rather than engine first?