I tend to agree that post counts are not really very important. Of course if Thomas weren't still way ahead of me I might well change my mind.
But I also view post count celebrations as sort of like birthdays - it's not so much that it's necessarily an accomplishment in itself (depending how much thought goes into each post) but it's an excuse to have a little party where we can show our appreciation for someone. The post count is an arbirary and flawed metric, but we don't have a better metric so readily available, and I see no harm in periodically saying "thanks for all the helpful posts" when someone crosses a milestone.
As for the original question: bug or feature? Well I'd say it's a bug really, but for the most part it just doesn't seem very important. It would take some (probably minor) effort to change it, and it would make the server work a little bit each time someone looks at a thread
if the server has to look up the current post count of each participant. And I think you're the first person to call attention to this particular behavior. Others have noticed it, but not been too bothered by it (I think). So unless there's a sudden groundswell of support for post count reformation, I think fixing this "bug" will remain pretty low on the to do list here. No offense, that's just how I see it. Thanks for the input though, we do appreciate feedback on this sort of thing.