first, post count has very little to do with the content of the posts. i very rarely add a "me too" to a thread just to get another post. heck, i didn't even post on the "post here and add one to your post count" thread. (great and funny thread, by the way, whoever the author is).
more often than not, and i speak from experience here, it has everything to do with the poster having an opinion and wanting to share it with others, either to reiterate a point with additional evidence or to refute/rebut claims with opposing information, and in some cases, to just be plain stubborn and refuse to give in. besides, there aren't a lot of people who have more posts than me, and all of the ones that i can think of right off hand are great members with (generally) valuable input into the threads. or in some cases, they consistently provide the comic relief for what many times becomes heated discussion (thank you lordtrader). and let's not forget that GD isn't the only forum on this site. more than a few have upped their post count in the other forums.
post count is/was also useful in helping to identify trolls.
solution: have both post count AND date of registration. [:D]