Part of the issue is getting people to wear them--properly. People like to pull them down below their nose. Again, there isn't a whole lot of data, but it stands to reason that only covering your mouth somewhat defeats the purpose.
And it follows that using a mask exclusively as a chin brace isn't helping anyone.
I don't follow this line of argument. Theoretically the concern is exhaled droplets which I assume primarily come from your mouth. If the point of a mask is to protect others and not yourself then why would the nose need to be covered?
Your nose has the same issue as your mouth in that regard. Remember, the virus is hanging out in your respiratory system; it stands to reason that the problematic droplets don’t necessarily originate in your mouth, but rather your lungs.
Even if that isn’t the case, your nose and mouth are connected, and both contain mucosal membranes. They’re sharing the same droplets.
In general, when you cough, sneeze, or even breathe, the same droplets are coming out of both your mouth and nose.
And it follows that using a mask exclusively as a chin brace isn't helping anyone.