They write some exact number of posts for a signature campaign, usually many of them answer only the first post in the topic or even just its title. If they'll start using notifier they'll won't answer anyway. Especially those ones who have a bunch of accounts. They are not really interested anyhow in what they talk about.
True, when you read carefully you will understand that some persons aren't really interested in what they talk. A close check will reveal that some even contradict themselves at the course of discussion.
The Original Post is very important. The first thing to do when you open a post is to read the first post of the Original Poster because it is the independent variable that controls the comments which now becomes the dependent variable. You cannot understand the thread without reading the first post.
Did the OP say he doesn't read Original Post before diving into discussion?