The thing is that in the current karma model, karma simply can't be treated as currency, because every time someone upvotes something, that karma is drawn anew from the aether, it's not a transfer of existing electronic karma bills or anything like that. You can't even say there's an infinite bank it comes from, because there's not. Karma currently exists only as a ranking, and although that's a goal in itself for some people, I'd say it's not for everyone.