If you start a trend, people will follow it. If you give people the ability to modify their DNA people will start copying each other, reducing gene pool diversity. What may seem like a trendy gene modification could quickly turn out to be disadvantageous, or even deadly. Imagine if everyone found it cool to possess the skinny gene. Skinny genes are currently very popular, especially among hipsters, but what if a worldwide famine comes? Of course it can become crazier than that, as there are trends with injecting cement into posteriors, plastic surgery on faces, injecting toxins into faces, huffing glue...