Genetics is a tricky business especially since if genes are too similar for too long the defects tend to pop up and weaknesses get magnified.  If certain genes are selected for their great effects and are used almost exclusively, the genetic structure will start to degrade and new defects will form, forcing people to change genetics after a certain time before the defects eat away at any of the advantages those genes give someone.  

Genes need diversity to effectively combat defects that start to appear in narrow gene pools.  If everyone starts to rely on a small set of super genes, soon humans will become too similar to themselves and unless they keep around other '[i]inferior[/i]' genes, genetic problems will soon take hold in future generations.  Then people will start to complain that previous generations used up all the good genes too much so now future generations can't use them anymore.

Humans are really good at screwing things up, so I certainly have a lot of fears of what could happen to the human race if genetic manipulation becomes mainstream.  There is a good chance some problem will pop up down the road that scientists did not realize would happen after manipulating DNA for so long that will become impossible to fix and potentially lead to the extinction of the human race.  There would have to be a heck of a lot of research done on the subject before I would even think about directly manipulating what effectively makes up a person, with human hands or even robotic ones.