Accounts with 0 posts are usually either spam bots, lurkers, or people who signed up and then forgot about the site.
Admins can always run a purge of accounts with 0 posts, though sometimes it's easier just to leave the accounts there as deleting bot accounts means the bot can just sign up again. And you never know when your lurkers might post, so why risk deleting some of their accounts?
You're not going to get an accurate number anyways. Lots of bots get 1-2 posts in, and lots of members make single posts, which will offset the numbers as well (over half the member list has less than 10 posts.) And then banned members don't show up in the regular member's memberlist (at least in SMF 2.02, I don't remember if they did in 2.0 RC4), so you're not going to get an accurate assessment on that front, either.