Because among your ideas for the membership were things like:
"Able to access some area’s that are visible to moderators only right now"
"A private member only forum"
If the "membership" is only extra perks that don't affect the forum interaction I wouldn't care but I've been on too many forums that have created limited access for some members over others and seen the us and them attitude that results. The blow ups, misunderstandings, and hurt feelings that can occur when someone is left out of the conversation is just not worth it.
There is also the risk of people who don't want to pay (or can't afford to) feeling unwelcome.