People cover their faces in public for a variety of reasons.
In parts of the Far East, when you have a cold you wear a little mask that covers your nose and mouth. This is done out of courtesy to others, so that they will not catch whatever you have. And in some parts of the Arab world, women cover their faces to protect their modesty.
There may soon be other reasons to wear a mask in public. We are rapidly approaching a time when it will be technologically easy for anyone walking down a city street to unobtrusively call up your name and identity in their wearable PDA, using facial recognition and web search. For various reasons, you may not wish to be so readily identified by strangers.
I wonder whether it will start becoming socially acceptable, if you wish to remain anonymous, to wear a mask in public. Not to prevent people from catching your cold, but rather to prevent people from catching your name.