Post count isn't nessecary, but it is a little fun. I know this sounds juvenile, but it is kind of fun to say "Wow, I just made post #(insert big, round number here)," so long as it dosn't become a contest. You could compare it to having a party to recognize a member of a community who has been around for years. Just as newbie threads are like house-warming parties. I wouldn't miss post count if it was gone, but I think that having postcount does add a little something to the community. And that, together with Morrus' much more compelling argument, is enough reason to keep postcount, I think.
(I really hope this wasn't about me, I know I made a lot of posts last Friday, coincidentally just before I hit 1,000, but my flurry of posts was just because I was in a good mood, having just landed a job, and becuase I like to make a round of the boards before I game, which I was doing Friday night. Sorry if someone thought I was just pushing for 1,000, in fact I didn't even realize I was getting close until I reached 999.)