"Value" is something that is held in high regard... something special. I think all people have the equal "potential" to be valuable. However, I don't believe all people are equally valuable.
There is no comparison when it comes to a "retarded patient" or a "doctor who could potentially save hundreds of lives." How can "value" be placed on either when looked at in this capacity. Obviously, someone who has chromosomal, mental, or or other handicaps won't be able to perform health care on "potentially hundreds of lives." However, he/she could provide joy and happiness to those same potential hundreds just by his/her smile or love for life. To try to equate these two is sort of ridiculous in my humble opinion.
Some people work very hard while others are sloths. Some people have a high moral while others don't take responsibility for any action. Some people have great compassion and utilize that in positive ways while others don't have any empathy and only work for their own needs and not the good of others. Those who make a difference in this world or within their own worlds (i.e., family, friends, co-workers, etc.) are valuable.