A full top is exactly how it sounds and becomes necessary when there is more scalp showing than hair. In order to perform Scalp Micropigmentation on the entire top, the hair needs to be shaved very low. Beings the newly created follicles are flat, having long hair on the sides and short on top would just look like you got your clippers in the wrong spot.
Letting go of what hair you have left can be very hard.
I often hear, "I'm just not ready for that". Remember, the goal is to make this look natural and undetectable. To be blunt, hanging on to hair on your sides when the top is bald, just ages you. Doing a Full Top procedure and softly fading into your sides is going to result in a cropped haircut. Not only is this look very modern, It can take years off of how you look.
Taking years off can be great but lets be real, age still matters. Creating a soft, age appropriate hairline is key to making your SMP look as realistic as possible.