The “a vote for a third party is a vote for Clinton” fallacy

Those who say they are voting for a third party or independent candidate often hear that doing so is equivalent to a vote for Hillary Clinton.

In this reasoning, not voting for Donald Trump is voting for Clinton.  This is because, I assume, a vote that Trump doesn’t get is helping his opponent win her majority.

But you could just as easily say that voting for a third party or independent candidate is also a vote that Hillary Clinton doesn’t get.

I suppose conservatives who don’t like Trump hear this more than liberals who don’t like Clinton.  The latter could be told that a vote for the Green Party is a vote for Trump.

But the assumption is that the conservative who doesn’t like Trump would vote for him, that he would rather Trump win than Clinton.  But that begs the question.  If the person thought that Trump would be better than Clinton, he or she would surely vote for him.  Choosing a third party is saying “a plague on both your houses,” that neither is clearly better than the other.

Again, we just have to vote for the person we want to be elected and let the chips fall where they may.

Or am I missing something?