It seems to me that subscriber counts provide social proof. If you want more subscribers, then building social proof will influence more visitors to subscribe. It’s text book Cialdini. Plus, when I see that a blog has a large subscriber count, I can assume that it’s worthwhile to at least put the feed into my “probation” folder :)
Of course, this doesn’t work until you reach a certain level of subscribers. That’s why I don’t have subscriber counts on my blog :) But I’m getting closer every day!