That is possible. Take a look at the category security settings page:
The Builders group would for example be the group that people pay subscription access for.
You are free to add the everyone group as well and remove the create and reply permissions. So everyone can see the posts (no paywall) but people have to pay to create topics and reply to them.
(You could go even more fancy and charge extra for people to create topics vs just being able to reply)
So this plugin can be used to create a newspaper like service where you have to pay to be able to write comments.
Or many other ideas are possible!