if your forum (download sections) are open to all including guests then yeah why not prune members with 0 posts but if its not they obviously joined to see content so I don't see why you should prune them as without those leechers there would be not much point in a forum? yes there are some members who download and upload but a lot of them just upload for cash, rs points, ect so to me it would deepened on if your forum is accessible all areas by guests
well my forum isn't accessible to guests, but still i stand behind that if they wanna come back they will.. then you will know who your members are and who isnt/wasnt.