It's not so much "worthless posts" as stuff that doesn't really add value to the forums and will slow down the migration. The less there is to migrate, the less time the site will be down. Stuff like Word Association games, Last Letter games - they can be HUGE topics, but don't really add any value. By getting rid of stuff like that along with things like I touch goats threads now, I can cut down on the amount of time the site is offline - which is only good for all of us.
As Phil said, post ranking will still be available after the migration, but I'm a firm believer in QUALITY, not quantity. Posting for the sake of increasing post count doesn't add anything in terms of value to the forums so post count will only be available by looking at a users profile. Virtually everybody will see a reduction in post count after the migration due to the current purge that's taking place.