For me, my issue is this:  If you were to have multiple membership levels and want to separate a post to make it visible to only Paid Members, its currently not possible without making multiple posts. Paid Level 1 (ie, billed monthly) members would not be able to comment alongside Paid Level 2 (ie, VIP billed annually) members.
But I believe Nabil's issue is he wants to have both 'Public' & 'Members Only' and Leonid was suggesting to replace 'Public' with 'Members Only'