It might not be wasteful if you use your vote to make a point to your children about how important it is to stick to your convictions, that merely capitulating to the inevitable can stifle one's desires to become a civil revolutionary. A journey of a thousand miles begins with a single step.
It won't change the outcome of the election, but it could have value in its symbolism.
Ya, a journey of a thousand miles begins with a single step, so you don't need to move backwards if you can't make a giant leap. Being a more progressive voice in a larger party and helping to get things implemented and you could tell your kids of a goal you accomplished instead of how you helped make things worse in order to make a point that nobody cared about.
For instance, if this election were close and it mattered whether or not one voted for a third party, would you want to tell your kids that you helped secure the Supreme Court for a generation and won them all the rights and freedoms that the ungrateful little bastards don't even appreciate, or do you want to tell them that it's kind of your fault that Philidelphia burnt down because you were trying to make some kind of abstract statement?
It's nice to stick to your principles, but if those principles lead to immediate negative consequences in exchange for the mere chance at a step towards long term positive consequences, then that's not a good trade off.