Well, the problem is, as always, where to draw the line. Things like that can get out of hand quickly. What is a disease?S.H.A.R.P. said:It's highly controversial, but from a logical point of view, it would almost be silly if humanity would not sacrifice a few people's need to reproduce in order to get rid of nasty diseases and genetical faults.
Is bad eyesight already a bad enough reason?
What about red hair?
I know that's not what you mean but it's what something like this can lead to. Think Gattaca, in the long run.
Also, things like cancer aren't really based on a single genetic cause, they're multifactorial. Meaning that if your, say, uncle has colon carcinoma, you have a higher risk than somebody who doesn't have a sick uncle, but it's still very low.
There are a few cases of distinct genetic disorders that lead to cancer, but they are an absolute minority and, to be honest, still very well treatable because of the known predisposition. For example, a person with adenomatosis polyposis (EDIT: different language, slightly different name) has a very high risk for colon carcinoma (almost 100%) but, since it's known, will be closely monitored and treated if necessary.
Those that regularily die from cancer aren't the people you keep a close eye on but average joes/janes with a certain predisposition.
Also, many/most cancer cases are completely spontaneous without any family history.