Oh - so watching the category does send out emails? That is still a better option that checking to receive emails for all new topics and posts and then muting categories. 
You could add a column for levels of each type with checkboxes for notification and/or email? That way a user could select what action they want to take on the category and set preferences right there?
There could also just be an additional setting in the account preferences to receive an email for every topic. (Underneath the one to receive an email for every new topic and post.)