There is a great need for that.
If want someone to subscribe to the forums category, how he will know what is inside if he can’t even see the category or titles of the posts.
For example I have instructions how to do something , they are free. 20% people can use it but remaining 80% because of complexity can not. I do not want to waste time with detailed explanation and specific questions. Who needs such level can subscribe and get it.
SMF forum has “view only boards mod” where everybody can see category and post titles then they decide if they want to subscribe. Only in that section support for particular product is provided.