It's a probability question, as well as risk to the wearer question.
If one is in an environment thick with a virus, you are right. But the risk of spreading infection (if it exists), or instead view as the "size and intensity of the virus bubble," and a mask has some effect.
Lots of variables. And you are right, these masks don't "stop all" spread of virus.
This is a good point. There seem to be some FReepers that believe that because a mask is not 100% "effective", then it is 0% "effective".
Mask are useless which makes me wonder why are mandates to wear a mask taking place.