I just wanted to weigh in on the natural selection vs. humans altering genetics/environment ideas that have come up in this thread.
They are both, at their core, the same thing. Humans like to think that we are 'above' or 'separate from' nature, and that the things we do are somehow not natural. But this is pure hubris. Every person reading this is as much a part of nature as any animal or tree or microbe. And because humans are a part of nature, everything we do or create is a part of nature (unless you'd argue that a bird's nest or beaver's dam is unnatural because it was created by a living creature instead of just happening). It's been mentioned that humans modify their environment to the point where traditional evolutionary pressures don't apply. This is true. This is also perfectly natural. Many organisms alter their environments to some extent. We just happen to be a whole lot better at it than most. And from the point of view that humans and all human works are natural, those changes in selection pressures are completely natural as well. If we start making extensive modifications to our own genes, might we screw ourselves horribly? Absolutely. Nature screws organisms over horribly all the time. There are a variety of natural ways that changes can be introduced into the genome. Humans deciding to make these changes is just another one.
Now all that being said, wierd certainly has a point that allowing severe genetic defects to spread through the population via treating the symptoms so that those who suffer from them can lead a relatively normal life and pass those defects on, could certainly have disastrous consequences down the road. Possibly leading to large die-offs of humans, or even human extinction if it goes on long enough. And everyone needs to decide for themselves whether or not they believe that potential future harm to the species/society outweighs the immediate needs of the individual. I'm personally of the opinion that the best course of action is "Treat the individual (to the extent that they desire treatment, and resources to treat them are available), but acknowledge the long-term problem that can create, and devote as many resources as feasible to figuring out a way to mitigate or prevent the long term issue".
I hope this is all coherent, because it's nearly 4am and I haven't slept yet.