Peter & Bette,
Everything is not planned. Randomness and chance play a role as well -- especially in genetics. Even when certain characteristics are programmed, others are left to chance. Chance adds robustness, vibrancy and spontaneity to our experience and learning opportunities -- it encourages creativity and adaptability. Life and circumstances unfold purposely and with many random bumps - ours is to deal with it the best we can and learn from it.
Sometimes, short lifespan and defects are planned -- and when they are they are arranged to benefit the potential growth of the system (everyone evolved -- child, parents, relatives, friends, doctors, hospital staff, etc,). Sometimes a defect provides a unique perspective and reality experience that is deemed useful (to both the individual and those in the individual's environment).
Typically, those beings with a higher degree of planning orchestrating their PMR experience packets are lower entropy beings who have already mastered much of what chance has to teach.