The "subtle" advantage of Sickle cell trait is enhanced Malerial resistance
Sickle cell is an interesting philosophical one for gene therapy as a radical interpretation (the sort of thing usually promoted by people who don't have it) is that it could be depriving people of African origin of their genetic heritage.
From my awareness of the symptoms I think most sufferers of the condition would say "F**k that, I've never been to Africa." Then again maybe some people might feel that life threatening (and often extremely painful) random blood clots are their birthright.
Now things get very murky if someone says "I'd like my sickle cell gene to be enhanced to give me the Malerial resistance without the random blood clots."
Another interesting one (courtesy of Vernor Vinge) would be if you could have the obsessive focus of autism on demand? Controllable by you and capable of being turned off.
I think most genetic diseases are just that. Defects that most people would want removed from their genome.
But such choices will come.