It's already not counting some of the boards (mostly the "Socializing & Games" and "Adult Socializing" boards). Technically, it could count the posts on whatever boards the admins deem appropiate (but as far as I remember, it doesn't readjust your current postcount if boards suddenly stopped counting, though I could be wrong here).
But then, what's the point? Even if you only count the right column for instance, then there's still OOC threads in there. And what about the one liners VS the paragraph posts. Does 5 oneliners really indicate more activity than 4 posts of 8+ paragraphs?
Also don't forget that E is more than just the stories being written on it. Removing the postcount from all those boards would indicate that those sections are not important, which is incorrect. Only the two socializing boards have some threads that could bump postcount up into insanity, and that just would be wrong. It would be as pointless as counting every line in the shoutbox and the chat.
As for GT's initial question, I really don't get the "instead of". Technically it would be possible to add the registration date next to each post, but that wouldn't have effect on the postcount. It's not like the postcount is taking up a specific space. Both postcount and registration date can easily be found in the profile, though, so personally I don't care about either being visible.