Think about your job. Depending on your workplace, it could be a huge disadvantage to get found out as a geek. You could get overlooked for promotion because you "play with toys" or "spend a lot of time playing World of Warcraft". Being a geek probably doesn't affect your performance in the workplace but it could get you shunned by people with prejudices against that sort of thing. That's an innocent example, there are far more complex and difficult examples which you are ignoring. What about if you are gay but you want to keep it secret from your family or you are living in a country where it is dangerous to be openly homosexual. I could go on listing traits, vices and attributes that could cause people problems.
Saying that someone simply "shouldn't do it" dismisses the fact that some things people wish to keep secret are part of their true identity. Someone shouldn't be forced to hide their true identity but it is an unfortunate reality that in many cases you must in order to protect your career, your family and even yourself.