Hair under the wafer can cause a number of problems. Ingrown hair, pulling when changing and think of it this way, the more hair the less skin is in contact With the wafer so the more chance of a leak. 

Before I started taking hair under my wafer seriously, it would grow quickly between changes (to the point where it would come THROUGH the tape border!) and it was always itchy or caused ingrown hairs. I also suspect it may be responsible for a few leaks since it creates channels under the wafer for stool/liquid to easily penetrate.