You know that's always a sore spot with users - they tend to be very outspoken if anything happens to their post count. (Hence why I shot off an email when it happened - wouldn't want the admins here to miss out on a user gripe! )
That actually is a quite nice mod.
In some ways it doesn't even make sense to reduce the post count when deleting threads. I mean, you have still made the posts.
So I guess it's a better indicator on how much time you've put down on the site if you don't reduce the post count.
Then, in other ways it makes totally sense to just show how many posts that are actually available. So I'm not sure what's best to be honest.
One thing I am sure of though is that post count truly is important for many.