Those cases where forewarned doesn't help are definitely at issue. The classical example is Huntington disease. It's an autosomal dominant death sentence and there is no treatment or way to alter the course of the disease. Some people don't want to know. There is actually a very elaborate three-phase commit for testing/getting results for Huntington disease, and geneticists won't perform the test on a minor.
On the other hand - in case of Huntington's there is a 50/50 chance of your children inheriting it from you if you have it. So it can be argued that is it not ethical to test yourself if one of your parents has it in case you are planning to have kids?
I can understand some people do not want to know and still have kids and are ok with that, however I would not be in the case of Huntington's specifically (no cure, very, very nasty disease - although depending on the repetition count of the gene pattern that causes the disease the age and severity of when the disease manifests itself vary).