If you understood how VB handles the member data base you'd see it takes up very little space. It's the threads that take up space.
This pruning is up to David. Since he's a nostalgia buff I doubt seriously if he would do any pruning. Besides whether or not a member is active the members numbers are what counts. Makes you look busy, grasshopper...
Dont Prune unless it's necessary. People are allways saying "You could have just used the search function" etc, and by removing members, you will either mess up the forum, or remove the posts too.