I believe I've seen this suggestion before, but I have several paranoid friends who would like to use deckbox but don't want the whole world seeing their collection. I think especially for people with expensive collections you may be missing out on potential members.
I'm not that paranoid, but I do think it would be nice if you could set things so people could see your tradelist and wishlist without necessarily seeing your entire collection. You can edit privacy for decks, so why not your collection?