Its not simply an issue of splitting the vote, its about making the parties WORK for said vote. If politicians realize your vote is already predetermined then both the incumbents and the challengers will not campaign to win your vote. And in Baracks case, you'll be completely ignored once you've casted your votes as well.
If politicians realize your vote is worth nothing they won't campagn for it either. Being a minority we can't cause a major shift by splitting up our vote, all it would do is reduce our bargaining power.