Voting is based on the assumption that one user can cast one vote on any given post. If you allow unregistered users to vote they can cast as many votes as they want, as long as they keep establishing new connections. Even if you were to limit votes by IP (which would incorrectly restrict multiple people behind NATed connections to a single vote), they could still evade that by using proxies or TOR.
Bottom line: allowing unregistered users to vote will be abused to hell and back, particularly by spammers and other bad actors.
