Someone I know created an elegant response to this "your vote doesn't matter, so vote for a third-party candidate in protest" rhetoric:
Black and Hispanic people don't have the luxury of a protest vote. The LGBT community doesn't have the luxury of a protest vote. Women who are pro-choice, or want someone who values equal pay for women, don't have the luxury of a protest vote. If Trump wins, they get screwed.
I agree that the US needs to move toward making third parties a viable option. But no, now is absolutely not the time to take that step. Trump is likely the worst major party presidential candidate in US history, and I'm not being hyperbolic. No experience, ignorance of the very basics of politics, habitual lying that makes Clinton seem like a saint, an unwillingness to take on the responsibilities of president, policies that openly court bigots, friendliness towards a dictatorial Russian government, economic strategies that are primarily designed to increase the wealth of his own company, and eagerness to commit war crimes. The threat he poses is too great to even take the chance that he might get in.