One of the principles that makes a hair transplant successful is being able to give the illusion of density over the area treated, blocking out the reflection of light from penetrating through the hair and off the scalp. Even with a natural full head of hair the shorter the hair is cut the more reflection of light off the scalp is visible, giving the appearance of thinning even if there is no hair loss; dependent on hair characteristics some more than others.
As hair loss progresses hair styling can become difficult to hide the light reflection off the scalp. One reason why many shave their hair to reduce the contrast between hair and scalp.
As we cannot replace like for like hair lost blocking the reflection of light becomes more important and a fundamental to achieving a natural looking and cosmetically acceptable result. Placing the hair at a definite angle and orientation dependent on the area of the scalp, temporal recession, and top of the scalp along with the density placed and matching the hair shaft curvature will aid the illusion.
It comes down to the law of physics; thick long hair will cover the scalp better than thin short hair. With thick, longer hair the light does not reflect back as easily and less scalp is visible; but we are not talking having to wear the hair very long.
Ultimately to block the light reflection and give the appearance of fullness there needs to be a minimum hair length; this allows each hair to overlap or cover the next hair more effectively. As hair loss progresses it is important to understand that it may not be possible to create an even density from hair line to the crown, and in some cases a lower crown density maybe required. Long term planning is important from the first procedure to ensure that the donor hair can achieve a balanced coverage; correct donor management and planning will allow even high NW candidates to achieve good coverage.
As the goal of a hair transplant is to block the appearance of scalp there needs to be a balance between the density of follicular units placed and the hair characteristics that ultimately determines how long/short the hair can be. The technique used does not impact on this aspect of hair transplantation; the responsibility belongs with the doctor making the recipient sites and the technicians placing the grafts in the recipient sites.