With the MIG is a one shot deal .You start the weld and your committed to the end. Starts and stops are more noticeable . With TIG you can weld one inch.. stop..reposition yourself .. and proceed and it will look like a continuous weld.
The appearance of the MIG weld is just not as nice as a TIG weld , In the end result , everyone wants a tight and professional looking weld that you are trusting your life to.