The idea of being able to read all the posts without visiting the forums might seem convenient, but such a feature would not be practical.
Consider this recent
Bendgate news thread. If each of the 589 users who posted in that thread were sent an email for each of the 3010 posts in the thread, the forum system would have sent 1,772,890 emails. Add to that the number of emails for people who would subscribe to the thread without posting in it.
Multiply that by the number of active threads and the forum system may find itself with tens to hundreds of millions of emails to send each week. And some would be to abandoned email addresses or sent to users who are no longer around.
The old Listserv style of emailing every message to every follower just doesn't suit a high-volume site like this.
The current system is both practical and convenient. You get a messages telling you which threads have had activity since your last visit, and when you return to the forums (which a single click) you can catch up from the post in each thread were you left off.