I am a strong proponent of privacy. That said, there are many issues to take into account.
I will not use locks because of danger. I will use hooks & eyes (both inside and outside) for privacy sake. The child uses the hook when they do not want to be disturbed and also uses the hook on the outside (up high) to keep nosy or exploring toddlers from entering.
The problem comes up when children share a room, rules need to set up between the roomates on how/when to use the interior hook.
Knocking and then entering is unacceptable in my book. You have no right to enter JUST because you announced your presence. Often, knocking opens a poorly latched door, thus the need for the hook.
Teenagers are a whole other story. In my home, my children were required to leave their door open when they leave and could close them when home. They never locked the doors because we never, ever entered without verbal permission, even when we had to wait for them to answer. I believe locks are never needed (except for the parents room) if everyone is taught, from childhood, to respect privacy.
Also, the rule was no hooks in or out after bedtime.