Do guest posts ever count?

Upon converting to a Member account, are his Guest posts added to his Member post count?

There’s no difference. Everyone has a post count, it doesn’t matter if you’re member or guest. The only difference is that guest post counts are hidden, but you can still see them if you look at their profile.

http://boards.straightdope.com/sdmb/member.php?u=87104

Click on the “Statistics” tab, you will see your post count among other information.

In other words, the things that happen when you pay for upgrading from guest to member are:

  1. No ads.
  2. Your title is changed to “Member”.
  3. More of your info is displayed with your post.

The posts you made as a guest and your post count are not affected.

Who uses the post counts? What purpose do they serve?

None.

By themselves they mean nothing, but in combination with the join date, you can figure out how active the person is on the board. The more active people are more likely to understand the culture of the board, as they’ve had to make more posts in deference to it.

At least, in theory. In practice, the only time I’ve used it is to determine whether someone is likely a spammer.