I think the reason why there isn't something like that already implimented here on the forum is because it would be a little too intrusive in a way, especially if the member in question whose posts are being followed, wouldn't have a say in the matter. Yes, everything posted on this forum is pretty much viewable to the public regardless but that just seems a little too personal.
If you're really that curious to see what someone has been posting, you could always just look at their profile and check their recent posts. Given, this won't give you a notification every time they make a post, but yeah, it's as good as you're going to get I think. ^^