Steve Shaw:
If one in a hundred of us is infected, then 99 in a hundred are wearing masks for no good reason and are not protecting anybody.

There may be many wearing masks to protect themselves from the 'one in a hundred' who is infected yet doesn't wear a mask. The one infected person in a hundred does not necessarily know that they are infected.
So the other 99 are not wearing masks for 'no good reason'. They are protecting themselves, and by doing so are are protecting others by avoiding some of the risk of catching Covid.