If you want to vote third party then evidently you are so fed up with mainstream American politics and culture that you want nothing to do with it. In that case, why bother with political parties? The theory behind voting for a third party is that somehow it helps a movement that later takes over a major party.

I think that misrepresents why people vote third party. It's more that you vote it as a sign of a movement's strength, which in turn helps that party to win matching funds and appear as a gradually more credible alternative. Even more importantly, it helps it with advertising for state-level offices where it can seriously grow.

If the party is large enough, sure, a major party might adopt some aspects of its platform. The Democrats did this for the Progressive Party in the 1910s-1930s. Or, it might replace a previous party that's too caught in internal contradictions to address the new movement's core issues. The Free Soil Party, later the Republicans, did this to the Whigs in the 1840s-1850s. And for the record I predict the latter for the Democrats. I don't think they can reconcile their increasing support from finance and the security establishment, a sign of the '90s New Democrats' victory in swiping these, with the growing progressive movement within the party. The Republicans also seem ripe for replacement, given that they're facing a demographic crisis on both age and racial grounds.

It's a very minor form of political action, and I wish people were more focused on being engaged politically beyond just voting for a new manager-in-chief every four years. The actual groundwork and grassroots movement-building usually matters much more, since that's how you pressure said figures. But it's a small and worthwhile action, especially if you live in a solid state where your presidential vote legitimately is thrown away by not voting your conscience.