Recent studies show that between 10 and 20 % of your patients may require extraction of permanent teeth. There are times when you must extract.
In my mind, there are four reasons you need to extract. The first is when you have severe crowding. The second is with a severe bi-maxillary protrusion and profile change is desired. The third is a surgical camouflage, where ideal treatment involves surgery but patient refuses and extraction allows a compromise. The fourth is when expedited treatment is needed and the extra time for space gaining would just not be practical.
Some of our ways to gain space are posterior expansion, distalizing posterior teeth or advancing anterior teeth and these may add time to treatment.