I would imagine they will offer a similar protection to a surgical mask, which also cannot "filter" outgoing air as there is no seal; it's just a line-of sight

I would imagine they will offer a similar protection to a surgical mask, which also cannot "filter" outgoing air as there is no seal; it's just a line-of sight protection, perhaps helped somewhat by absorbing moisture droplets. A piece of tissue paper inside the valve would probably have the same or better effect, if you could persuade people to do that purely for the benefit of others.