It's really no different when a third party enters the scene. If you don't vote for the candidate you agree with, then you are wasting your vote. You are sending the message that you want something other than the candidate who most agrees with you on the important issues. We are talking about an election, not a horserace. In a horserace you win only if you pick a horse that does well in the competition.
Elections are very different.
The object is not to pick the winner, it is to pick the candidate you most agree with on key issues. If not, then the winner of the election needs to know the extent to which voters really preferred other ideas. The winner will act differently after the election if voters send that message. Voting for anything less than the person or party you most closely agree with sends the winner the wrong message. Yes, but voting for someone who really can't win this election is wasting my vote.