You are correct about being in the primary season. One should find an electable candidate with the best positions and support him/her. I have always believed that, and it makes sense.
However, when the primary season is over, the choice will narrow down to two people. There is no 3rd party that has done the work to have a prayer in this next election, in my opinion.
At that point we get to make a decision between voting for one of the 2 candidates who has a chance of winning or casting some symbolic vote for someone who has no chance of winning.