ranks and posts help symbolize how long somebody was on a forum.
Personally i detect trolls by low post count and people that i can trust by big post count.
Your looking at it wrong them. Yes post count symbolize time spent on the forum, but since you liked my last equation so much, Im going to make a new one:
Quality > Time
Theres some people that have been here for a week or two, have triple digit post count, and I don't think they're dedicated or the best for the community. Yet there's some people in the exact same situation who I think will make great members. One member that has a unique situationis Swert. He's been here a while, but doesn't have a high post count, yet he has made a few great post, and the one day me and him had a great argument/ disscusion.
The point im trying to make is low post count means they're new, or have a busy life.