In terms of having any impact on a national election, even in a close state, voting for the front runners is the same as not voting at all. Or to use your words, voting for the two major candidates has exactly "the same impact as not showing up at all to vote for this office."
And voting third party has the exact same impact, and so does writing in a candidate. In all of those scenarios, the voter will have zero impact on the election. No national election has ever been decided by a single vote. So if you vote AT ALL you are throwing away your vote by any reasonable definition, because it is meaningless in terms of actually impacting the election.