The Constitutional form of government in the U.S. tends to stabilize into a binary, two-party system. The two-party system requires those two major parties to form coalitions that will attract the support of various groups and interests in order to win.
While it is true that third party candidates have essentially zero chance of winning, voting for them still sends signals that the two major parties consider when trying to form a winning coalition for the next election. If a significant number of people vote for McMullin or Johnson or Stein, it sends signals that affect party platforms and positions in future elections.It is perfectly reasonable and honorable to vote in order to send a signal for the direction you want the country to go, with an eye toward influencing the party coalition calculus of future elections, even if the candidate cannot win the current election.
But keep in mind that very little information is communicated in a vote. While you may be voting for a candidate you disagree with for complex reasons, your vote doesn’t communicate those reasons. In the aggregate, votes simply communicate that a certain percentage of people prefer one candidate over the others. So when the parties look toward coalition building, they probably don’t see your complex calculus and clever strategy; they just see a candidate and his or her platform with a certain amount of support represented by votes.
So a vote for a candidate you prefer may send a clearer signal than a strategic vote against a candidate.
If our constitutional system is working properly, then the checks and balances should theoretically prevent a bad president from doing too much damage. The fact that people on all sides see the election of the opposing candidate as potentially catastrophic suggests either that our constitutional system has already fallen apart, and that the president cannot be constrained by other branches of government, or that hyperbole is being used to manipulate people into voting contrary to their inclinations. Perhaps it is both.
Heaven help us.